In this situation I would pay close attention to the noises the pump is making. Is it dead silent(rotor not moving at all)? is it way louder than it should be? or does it have the normal pump 'hum'?
As Luke said, you should be able to get the pump out and disassemble/scrub it with a brush. if it's clean and STILL having issues, just try turning it on and back off. Pumps sometimes don't start spinning in the right direction (as funny as that sounds) and there are mechanical "stops" to make sure it's spinning in the right direction. sometimes (on cheaper pumps/powerheads especialy, I've found) it doesn't start properly, and a quick powercycle gets things going properly again. (experienced this with a cheap Sunsun powerhead)