unplug your RCAs and see if the amp still comes on. if it doesnt turn on anymore, you had a bad ground for the amp and youve now blown out the RCA output(s) on your headunit. if the amp still turns on, its probably just that the amp is bad.
also in responce to the above post, you cant really check the nominal impedence of a sub with a VOM. the subs impedence will vary dramaticly with depending on the frequency and the box its in. if a sub is REALLY blown (like the voice coil has melted down), you will get an extremely low resistance because it has created a short circuit. that said... if you decide to test a sub with a VOM, it usually comes out close to the rated nominal impedence. just didnt want anyone to get scared and think thier subs were blown because they got a freaky ohm reading
