Depends on the amp, but
heat from being driven too hard for too long, or inadequate ventilation/too much environmental heat
too low impedance from too many speakers hooked up
short circuit across the output, like from speaker wires shorting
something broken
are common reasons.
Amps usually reset themselves once the condition that caused it to go into protect is gone, like say it's too hot and then it cools down. If it doesn't it's probably got a problem that needs fixing.
Check the manual for your amp.
