Is it possible you have the rca connected to a set of preouts you have the HU faded away from. Or if you have a sub output you make want to check and see if disconnecting the power of the HU didn't reset the settings and now your sub out is turned to off.

If your amp is lighting up you have a good + - and remote. So all you have to troubleshoot is the signal coming from the HU and the signal going to the speaker(s).

Let us know if you need anymore help.