Make sure that the electrical cord is unplugged from the wall receptacle.

Remove screws on the side, back or bottom to separate the cover from the chassis.

Inspect the internal end of the electrical cord for looseness or damage caused by being pulled (a common problem) and replace as needed.

If there is no power, but the cord works, look for an internal fuse. Test the Fuse and replace as needed.

Use a can of compressed air to carefully blow away dust that is attracted to electronic components. If excessive, use a vacuum cleaner with a plastic tip (so delicate parts won't receive static electricity) to clean the inside of the unit.

Use electrical contact cleaner to clean the input jacks and speaker terminals. If one of the input jacks is disconnected from the wires, either reconnect or resolder it.

If none of these measures solves the problem, seek professional help. Modern audio equipment is built using delicate components that cannot be repaired; instead, they are replaced. Your audio professional will know which ones to replace, where to get them, and how to do the job.