Did you check the 'remote' cable already...
There should be a thin wire, probabley somewhere close to the RCA cables, going from your head-unit to the amp. It's used to turn on the amp along with your head-unit... Check to see it didn't come loose. And if it isn't then measure it too see it gets a voltage when you turn on the head-unit
Make sure you checked all the fuses. Most ice installations i've seen have a fuse both on the beginning of the power line (close to the battery) and fuses in/on the amp
