Try looking inside and see if there are any fuses and check them. A lot of amplifiers have rail fuses -- one for each negative and positive voltage rail -- to protect speakers against a transistor short. Usually, if only one side blows it results in a DC voltage on the output and that voltage is enough to trip a DC protection circuit.
Another thing to check is the thermal switches on the heat sink. Follow the wires and see if the connections have come loose along their path. The thermal switch breaks the circuit on overheat and activates the protection circuit. A loose connection emulates a thermal trip.