1. verify there is power at positive wire feeding amp from battery. If no power, check breaker or fuse out at battery on that same line. If bad, switch breaker or replace fuse.
2. verify the ground to amp is good. Use a volt ohmmeter if possible.
3. If you have power going into the amplifier and the amplifier power fuses are not blown, you have an internal problem that could be a lot of things. Like a rectifier, but this is just one of many possibilities. Have a qualified technician repair it. What kind of amp?