Check the fuses. Start by looking at the back panel. Then remove the top cover and follow the wires that go to the power cord and see if that leads you more fuses. You need to check inside and out because there can be more than 1 fuse.
If you don't find any bad fuses on your amp, check the fuses in your preamp the same way. Its possible the preamp is damaged and not the amp.
If you don't find any bad fuses on your amp, check the fuses in your preamp the same way. Its possible the preamp is damaged and not the amp.