Elisa Lam Died of Hypothermia (A theory)

The Elisa Lam case, for anyone who might not be aware, is a case wherein a girl from Canada was traveling around in California in 2013 and stayed at the Cecil Hotel (a hotel that is, I guess, known for being haunted and having bad things happen there)

"Built as a business hotel in the 1920s, the Cecil fell on hard times during the Great Depression of the 1930s and never recaptured its original market as downtown decayed around it in the late 20th century. Several of Los Angeles's more notable murders have happened at or have connections to the hotel: Elizabeth Short, victim of the Black Dahlia murder, the city's best-known unsolved killing, supposedly made the Cecil her last stop before her death, and in 1964, Goldie Osgood, the "Pigeon Lady of Pershing Square", was raped and murdered in her room at the Cecil, another crime that has never been solved. Serial killers Jack Unterweger and Richard Ramirez, the "Night Stalker", both resided at the Cecil while active. There have also been suicides, one of which also killed a pedestrian passing in the front of the hotel. After recent renovations, it has tried to market itself as a boutique hotel, but the reputation lingers."

and while staying there, she went missing and was later found deceased in the then contaminated waters of the hotels water tanks on the rooftop after guests complained of the water being black and having a weird smell. There are many theories surrounding this fascinating case and this is mine.

This is the video of her in the elevator:

She comes off as paranoid or anxious, or both. She's hiding, she's freaking out, maybe she was having a "psychotic" episode? Whatever the case may be, she was clearly trying to avoid or get away from.... something. (Ghosts? The hotel was/is haunted. Someone?)
The hotel seems like it had a strange effect on her throughout the entirety of her stay, she even had to be moved to her own room from a shared room after her roommate complained of "odd behavior".

It might make sense if it were ghosts. She was mentally ill (bi-polar/depression), and, IMHO, I really think that ghosts can make that worse in a person. They can mess with you and make you feel crazy which can sometimes then make you go crazy. (I would not be shocked if the ghosts of the hotel had negative effects on at least some of the guests.)
It's important to note that this was NOT how Lam would normally act, not outside of the hotel, anyway, which is why this case and her behavior in the elevator is so alarming.

"Outside the hotel, Katie Orphan, manager of a nearby bookstore, was the only person who recalled seeing Lam that day. (The day of her disappearance) "She was outgoing, very lively, very friendly," while getting gifts to take home to her family, Orphan told CNN. "[She was] talking about what book she was getting and whether or not what she was getting would be too heavy for her to carry around as she traveled.""

So, rather than taking the elevator, since it clearly isn't going anywhere, she chooses to abandon it for going down the hallway, where she could possibly have made her way towards the fire escape, a direct route up to the otherwise inaccessible hotel rooftop.

In the following video, an individual shows how easy to get up there is, and he even shows that two of the tanks were left open (which, even without this case isn't a good idea anyway, because something could wind up getting in there and contaminating the water within, which is obviously what wound up happening):

I will include a comment in which someone translated what the individual was saying.

"Start from 00:23 We travel to Cecil Hotel trying to discover the secret hidden here with fear and expectation. This is LA. The place gathered huge attention after Elisa Lam's death. Cecil Hotel was splendid in the past times, but now it's only notorious after the well known murder - death of The Black Dahlia, also followed by some other mystery suicide. Now we are here, exploring the hotel, try as hard as we can to restore the truth we believed. 

00:51 The hotel isn't overcrowded. On the contrary, Cecil only has few guests. Walk through the hallway you can feel unusually cold. Some words of creepy description on the hotel's terrifying atmosphere simply are not groundless. We will explore the hotel start from the main hall and then to every corner detail. We don't do this film to satisfy our desire of playing detective. We are showing every one a clear acknowledgement of what kind of a hotel the Cecil is. From that we should avoid wrongfully misunderstanding and supernatural thoughts.

01:19 Geography position - the environment. Cecil was founded at the central of the LA city in 1927. The north is Glendale. The east is Chinese area. The south is Wellington Park. The west is Hollywood. The hotel is a "E" shape building. It has three main floor which were connected by a hallway. We can see the hotel's front, flank and back side. The following are on-the-spot investigation. The back side has a fire escape stairs 01:57 Except main hall, the other parts of the building are old 

02:04 Enter the hotel's main hall. Tips: If you are scared, turn off the page and leave. 03:08 We start from the emergent exit of the hotel's front side. The hotel has three hallway like this. This is 13th floor, For western manners. It is marked as 14th floor. 03:25 Now I turn into the main hallway that linked the three branch buildings. The elevator is right on the way. The right hand by me are public rooms such as bathroom or toilet. That means every one can get in or out. 03:40 Now we see the two elevators and the mirror. 03:45 The right side are stairs which can lead to 15th floor. Pass through the door at the 15th floor will lead to the rooftop - only hotel's staffs can open it. 03:46 The elevator won't move no matter how you press the button, until somebody press the button from another floor. 03:58 We reach the end of the third hallway. Above are details of structure of the building. 05:33 Two blind spots where you can't see while stay in the elevator. 08:08 I have no idea where the double wing doors lead to. It is a different path from we saw before. 08:57 Throw the body into the tank from here is easier than carrying it while climb the stair on the tank."

They show how easy it is to not only get to the rooftop but to climb up even higher to where one can actually get up to the top of the water tanks and then climb down onto them (they did not climb onto or into the tanks in the video, obviously) but I do believe that it shows how plausible such an action would be, and fear and paranoia can make people do some crazy things to get away from whatever it may be that they're avoiding or scared of.
I do not know if they could have been open when she went up there (it doesn't seem unlikely that they might have been), but should they have been, she could have chosen to attempt to hide within one of the tanks in the midst of her paranoia and fear, wound up getting trapped due to having no way to get back out and wound up getting Hypothermia (Hypothermia is a medical emergency that occurs when your body loses heat faster than it can produce heat, causing a dangerously low body temperature. Normal body temperature is around 98.6 F (37 C). Hypothermia (hi-poe-THUR-me-uh) occurs as your body temperature falls below 95 F (35 C)) which would explain the fact that she was naked (often in the last stages of Hypothermia, the individual who has it might feel hot to the point where they start stripping off their clothes to feel relief. Plenty of Hypothermia victims are found with little to no clothes on.) This is all going based off the thought that the water in the water tanks were cold, that the dark interior kept the water cold and thats why she would wind up with hypothermia inside one of them.
Her autopsy "revealed" her cause of death to be accidental drowning, and perhaps that is so, but I feel like in some capacity hypothermia had a role to play in her death.

As for her missing phone, I feel like maybe she had it on her earlier in the day but wound up dropping it somewhere in the midst of all this craziness to which someone then found it and kept it?

These are just thoughts I had while reading about the case, therefore this is just a theory. Obviously and unfortunately, I don't think anyone would be able to really know what happened unless they were there.
Whatever the truth may be, this is a sad case and my condolences do go out to her family.
I would love to hear your thoughts in the comments, should you have any.