Protect

Protect mode would indicate that the system is shutting down due to a fault in the output which drives the speakers.

You can check the speaker wiring, are there an loose or frayed wires? Any small "whiskers" or wire touching anything? Check the wire all the way to the speaker for damage, some pets love to chew wires!

Are you using the original speakers or added extra? The impedance of the speakers must be in the range shown on the back of the unit.

Have you perhaps had the volume up very high? This could result in speaker damage that could also cause this error. Try disconnecting the speakers & see if the unit powers up, if it does then put the speakers back one at a time to discover the faulty one.

If all this checks out OK then the output stage itself is probably faulty & that's a job for the repair shop.

Please update the question & let us know if the information given was useful to you - Good Luck!