Two possible causes come to mind:
1) stalled ( seized ) motor.
2) You have a short.
That said, without further info, you likely will get no further answers... Tell us more. Make/model is real important, as is what type of system ( Vacuflush, elec head w/ raw/pressurized water flush )
The more you tell us, the closer to the problem we can get. Otherwise we are just plain guessing.