As reported exclusively by russian sources at the moment, he lost consciousness after a walking hour and prison medics were unsuccessful in reanimating him, as per sources in УФСИН (government body regulating prisons and punishment). He was 47 years old at that time. The last time he was heard of he was moved from Moscow-based prison into the IK-3 named Polar Wolf, a penal colony located in a permafrost region near the town of Harp, where he found his end.
No other sources commented on that by now. At that time, there’s no independent proof of that or other explanations but the one given by prison authorities.
A fitting reminder is that presidential elections are to be held in 15-17 of March, meaning it happened exactly one month prior to them.
Navalny was a buttjoke in Russian politics. The only time he gained (some) relevance is when Western media was propping him as Putin’s “liberal” rival.