Check to see if the fuse has tripped. Is the power plug plugged in? Plug the pump into a different socket and check to see whether it works.
The suction line may be leaky, or there is air in the water. Check whether the backflow preventer is present in the suction line and that it is working properly.
Check to see if the fuse has tripped.
It may be dry running; the suction line must be completely filled.