If either the protect light has come on or the sound is not coming out of an amp can be a multitude of faults. We always say to people who call us up and say this to us and ask how much it will be that is is like calling a garage and telling them your car has stopped and asking how much it'll be to repair.
Most amplifiers have a protection circuit that is designed to stop harmful DC voltages from burning out the speaker or damaging the amp further.
A cause of the protect system to kick in could be overheating through overload or blocked vents, a speaker load issue (wrong impedance cab used), a genuine internal fault, or rarely a fault of the fault detection circuit (yes we've had these!!)
Things you could try to see if the fault goes
The first step would be to check that the fans (if any) are working and that the amp is free of dust and rubbish. Sometimes an amp in a flightcase can overheat where the grills have been blocked.
You could try removing the speaker load, and either testing the speakers on another good known amp or check another set of speakers on the amp that is tripping.
How you can help us in initial conversation about your amp not working.
Tell us as much detail about what lead up to the problem, was it working all night but was really hot to the touch? Did the amp or speakers start making a weird noise? does it come on for a while then go off? does it only go off above a certain volume?