Most aircraft have atleast 2 packs. A dual pack failure or other situation requiring both packs (or both engine bleeds) switched off is unlikely.
In this case, the outflow valve will move to the fully closed position in an attempt to maintain cabin pressure. The aircraft will tend to depresurise as they are not air tight. How fast it depresurises will depend on how much it leaks. It is cabin altitude that will rise, not cabin pressure.
Initially cabin temp would fall with the drop in pressure, although later will heat up quickly if the cabin is full of people and no other action is taken.