Unplug the power cable of your pump, but keep the plug end out of the water.
Remove the pump from the pond and unscrew any plastic attachments connected to ports at the top of the unit.
Check the top ports to see if they are clear of debris and gunk. If they are blocked, remove the debris.
Check the intake valve at the bottom of the unit to see it is free. Remove any debris and gunk from the valve.
Place the pump back in the water, but do not place it in the center of the bottom of the pond. Place it close to the sides of the pond. Leaves and other debris tend to congregate at the center of the pond, and this can be your problem in the first place.