How many channel amp is it?
Did you check your RCA outputs from the radio?
Quick check is to take the RCA cable from the head unit, while the radio is playing, tap the rca against the input on the amp. If there is static, a pop or squelch. then the amp is doing its job.
Start at the source.
If not I do have a place that repairs amps. They are in Mt Vernon, and is a good friend of mine.
Good luck.