It is possible that the amplifier had encountered a fault with a higher current drain which had triggered it to a protection fault condition. The fault can be your output drivers- the MOSFETS/IC's fitted for both the channels, Use a meter after disconnecting to check for short in the drivers. Disconnect the positive and negative voltages to the output and see if the protect changes. Even a fault in the preamp stages that drives in high current into the output can shut the Amplifier. Sometimes this can be a noise like a HUM or HISS before the protect works. Faulty capacitors in these circuits also can cause similar issues and needs close observation.
You need to confirm and replace the specific stages or outputs. If not there can be issues in the mother board, maybe the protect circuit by itself is shutting off due to a faulty bias , maybe a leak in any voltage/current sensing circuit. Also disconnect the speakers and test, if the protect is off then check for short on the speakers.

Finally probing using a multimeter to check for short in circuit is only a preliminary step. Since there are densely packed board it is beyond a certain limitiation to check on component level fault as the board replacement will give a faster result to converge on the section.