Puzzles/Lateral puzzles/7/Solution

Possible explanations:

The cell contained nothing, not even oxygen, so the man had asphyxiated.
The cell is a coffin, the man was dead before he was put into it and buried (and he was naked because he was a naturist).
The man was in a mental institution where he took a fatal allergic reaction to medication he was prescribed - or he did not swallow his medicines at the prescribed time but saved it in order to overdose.
The man was in a mental institution and the temperature drops. Since he was naked he died of hypothermia (this actually happened in a real hospital once)