Disconnect the speaker wire & input cable from the back of the amp then try turning it on.
If the amp comes on normally then turn it off and reconnect the speaker wire one channel at a time and try it. A shorted speaker wire or bad speaker will cause the amp to go into protection. Then you can try the input cable to see if that is the problem.
If the amp still goes into protection then you will need a technician to troubleshoot it. Problem is inside the amp. Good luck.