Originally Posted by DMiner
Sounds like the pump is frozen. This will produce a hum as the motor is trying to turn, unsuccessfully, but because the motor is frozen, no water is flowing.
Agreed, did you try smacking it?
If that doesn't work you will have to disassemble it to see if there is something stuck in it. Some of them have a rebuild kit available . But, they are quite cheap to replace if you do the work yourself.