hi there,
well u also have a standby mode and if that also fails there is the posibility to control the position of the outflow valve ( and therefore regulate pressure) via a manual AC or DC motor.. but theoretically speaking if it did get stuck in the closed position then u also have the option of turning on and off the pack to increase and decrease the amount of air being pumped into the cabin ( not sure if this is recommended) plus u have a pressure relief valve which will stop the differential pressure from exceeding 8.65psi so i don't thing u will be in too much trouble if what u described happened