If that valve is relatively new, it has a pressure balance spool valve in it. That is sometimes (always?) separate from the cartridge. If the water was turned off on one side, it can jam the spool valve to the other side. Normally, it'll slide back when both supplies are on, but may not if it is old, or there's some debris in there. This could shut off one side or the other, depending on which supply was on and which was off. Sometimes, a wrap with a rubber mallet will free it.