You may have to unplug it from the wall receptacle and let it sit for awhile; there may also be a reset procedure that will set everything back to factory defaults (check the manual).
If the receiver will turn on after that (with nothing at all connected), then start with just two speaker wires and try the tuner. If it works, then start connecting other devices and speakers one by one. Step by step troubleshooting may help you to isolate what caused the protection mode.