could be a number of things - you need to connect the amp to a headunit you know to be good and work from there backwards(meaning if the amp makes sound on a headunit you know is good then check the RCA cables you ran, then the amp itself, sub wiring...)

Also check to see if the amp has a protect light. I once had an older viper amp that had the protect light as red and the power light as blue and it would come on red and I thought It was on because I had never installed a viper amp. turns out the fuse was blown and it would immediately turn red so I never knew it was supposed to be blue...