As in all modern aircraft except the B787, bleed air is taken from the engine compressor and passed through the air conditioning packs. It is then fed into the cabin.
The control of cabin pressure is by varying the amount of air leaving through the outflow valves.
If you shut down one engine bleed or pack then less air is coming in so less air is let out.
If all packs are shut down then the outflow valves will close to hold as much air as possible but due to leaks the cabin altitude will rise and pressurization will be lost.
Depending on the size of cabin and leak rate the un pressurized rate of climb in the cabin may be 500 to 2000 feet per minute so a descent to lower level is required.
Hope this helps.