its a boss... its dead...
they blow like a hungry hooker man... ive blown more boss amps then all others combined... no shit...
On the off chance its not f**ked, do the normal checks...
you say it has power already, it might be in protect, let it cool down...
you might have disabled the sub pre out on the head unit... maybe...
check RCAs and what not...
check your signal wire from the head unit... if that has fallen out, grounded or broken, it wont know to turn on...
if its under warranty, go get it replaced (they dont bother fixing it most times) otherwise, go buy a better amp... please...