I'm not trying to horrible but you should know the answer to this one. This should have been covered in your type qualification training. I feel your answer will lie in the limitations sections of your AOM. As to what happens when the APU can longer supply pressurisation will be answered in the air-conditioning/pressurisation section of your AOM. But it is really worrying such a simple question like this is placed on the internet by a "qualified" pilot.
I'll also pose you another question. Can you do a packs-off take-off with an inop APU?