This was my second death to a mechanic I either didn't understand or was bugged.  My oxygen production was right next to my generator.  When night fell, it stopped producing.  The generator still had fuel and other things were running, but oxygen was kaput.  So I just waited to die.  If there was a reason for this, the game went to pains to keep it from me.  Not recommended.

Do you remember if the light was still on in the Air Cleaner?  and the little fan running?  

If you were standing inside, and none of the modules had the "no power" icon on them, then the Air Cleaner should be able to make enough oxygen to keep you alive and slowly fill the base.  But maybe there was a bug where the air cleaner thought it wasn't powered for some reason.  Not sure.

See if you can grab a screenshot of it next time if possible?  Or if it always happens to you with a particular saved-game, zip it up and email it over to me and I'll gladly take a look (steve@cairn4.com)