Cryosleep

The thing that the company used to sleep over 100 years.

The Cryosleep was perfected by Hold the Future in 2115, it was not something that they shared with the rest of the world however and actively worked to make sure no one else was able to. They managed to achieve Cryosleep with the strong lean on sleep, people would not be fully frozen and instead cooled to just before the crystallization of cells. The cells are able to still function and go about their jobs, so ageing still can happen but if done properly will not. Chemicals and a range of hormones are used to keep the body functioning through the years, with the added cooling of the body less would be needed then a normal sleeping person.

The chance of waking up is slim but not impossible, such a wake up would have to happen with a range of faults. That doesn't even take into account the chance that you don't stay cooled and age to death inside. All in all, death can happen, from ageing to loss of oxygen to simply waking up and dying in the complex from a range of things.