If the speakers are all good and wiring too, then it's likely that a fault has developed in the main amp section. Till this is cleared the protection will not go away.
You may be lucky enough to find a rubber band that fits. But the quality rubber in a household rubber band is not as good as a belt made for the purpose, and variations in stretch may not affect your broccoli but may be audible in a cassette player. Watch tension and don't put on too tight because it could hurt delicate bearings. I have repaired many items this way also. Bon appetit and good listening! HOW TO REPLACE CASSETTE DECK BELTS INEXPENSIVELY OR FOR FREE
if the receiver turns on and operates then eventually turns off due to protection mode its because a wire is shorting out the receiver.if it doesn't turn on or goes straight into protection mode then its probably time for repairs or a new receiver.