It sounds like you may have lost the main 15 amp fuse inside the amp. If that is the case, the amp won't power on at all.
Make sure the amp is unplugged for about an hour before you open 'er up. This will ensure that you don't get zapped by a power supply that is not fully decayed. Follow the power cord going into the amp and look for a fuse at the bottom of the case.
If you change a fuse, make sure to use the correct fuse, i.e. slow blow with slow blow, quick blow with quick blow. The ampere ratings must be followed as well.
Do you have a multimeter that can be set to ohms? If so, you can test that main fuse. Hold a lead against each end of the fuse. The meter should read about 0. If it's bad, the number will be very high.
Good luck,
Jerry