I think it's up to where you happen to be when you loose you're GEN's because if I remember well, the outflow valves will stay in the position they are in, unless moved in Manual Mode. So if you forget about your pressurization, the worst thing that can happen is that either your negative pressure relief valve will open, or your overpressure valve will open.
Since you will probably descent to land, and the outflow valves will probably not be fully closed, I expect you to catch up with the cabin altitude rather than the other case. If needed, the cabin altitude warning horn will sound anyhow, and you can gain control of the cabin, so no harm done...
Please coorect me if I got it wrong.
FT