Looks like y’all have it figured out what must’ve have happened, one of either two things (both of which have happened to me, though fortunately I noticed before virtual asphyxiation).
Novice to novice, same advice as already given. As I understand it AMFU are unnecessary power draws until you need them, so keep them switched off. If you do need them, unless you’ve bags of excess power your ship may power down other modules, in order of priority which you can set in the module tab; keep essentials such as life support on priority 1, and drop anything situational like SRV bay, limpet controllers, weapons, cargo hatch etc. to lower priorities. You can set several priorities levels, so then the ship will power down the lowest priority modules first leaving life support active.
Finally, and don’t worry because this has happened to me too, if you use your AFMU to repair a module it will power down, and with a couple of exceptions I’ve noticed WILL NOT automatically power them back up. For some reason life support falls into this category, so make sure to reboot it manually after repairs!
I’m sure this is all information that has already been imparted to you further up the thread, they’re a helpful bunch of space nuts here, but don’t let mishaps like this sour the experience. I’m still finding myself somewhere near the bottom of a delightfully long learning curve, and figuring things out the hard way is part of the fun!
Keep flying CMDR, and safe travels o7