What happens when the serpentine belt comes off of a 2004 Toyota Camry?

At the very least everything the belt powered will stop. This means your water pump no longer circulates the coolant so the engine may overheat. It also means the alternator will stop, thus the battery will no longer be charged and at some point will be too discharged to continue to supply electricity to run the engine, much less lights etc. It also means the Power Steering will stop, the car will be harder to steer.

There are probably other consequences I have not mentioned.

So the long and the short of it is put the belt back on or replace it if it is broken.