check the connections to your wiring harness, and your amp, and your box.
test them if you have a tester, make sure the connections are complete everywhere, and strong.
if everything checks out.... then you may want to take a look at your fuses, or you alternator, not sure. but that's a good place to start.
also, sounds silly, but make sure your wiring harness is completely inserted into the back of your deck.
also check the settings of your amp and make sure nothing has changed from when it was working, if you know what to look for.