Have you taken it apart and cleaned out the impeller and housing? There's only one moving part on the Aquaclears and I've found them impossible to truly break. If the gravel chewed up the impeller vanes then you can get a replacement impeller at your LFS
. Most likely it's just jammed. I had one stop on me, took it apart and there was a hair wrapped around the impeller...unwound the hair and it started right up.