Submersible pond pumps fail. The two biggest reasons for failure are not keeping the pump clean and lifting it by its cord. If your pump has stopped working or has slowed down a great deal, perhaps you can find out why and repair it. Or perhaps you will learn that you cannot save it an must buy another pump.
Finding the pump problem
First unplug the pump and lift it out of the water. Do not lift it using the cord. If you have no other way to lift it from the pond, tie a rope onto the pump handle and use the rope to pull it out.
Unscrew the hose clamp holding the tubing on the pump. While the tubing is undone, blow out the tubing just to get the pond gunk out. Turn the hose off and on to remove the gunk more effectively.
Now, turn your attention to the pump. If there is a screw on filter on it, unscrew it and blast it clean with the hose. Just under the filter, you will see the pump's impeller. An impeller is just like a propellor, but sucks water in instead of pushing it out. Use your screwdriver to turn the impeller. Does it turn freely? If not, try to loosen it, again using the screwdriver to turn it. If there is debris caught in there, remove it with the garden hose or keep trying to remove it with the screwdriver. If you can see the debris, try to get it out with your hands.
Put the pump in the water and plug it in again if you have the impeller clear and turning freely. If the pump does not run, pull it out of the water and check the full length or the electrical cord. Has it partially detached from the pump? Are there loose wires? Can you see insulated wires, but they are not covered by the black insulation that covers the entire cord? Do not try to repair the cord. A pump operates under water and you do not want to mix water and electricity. If the cord is stripped or broken, throw the pump away and buy a new one.
If your pump is running intermittently, the impeller is turning freely and the cord is intact, you have a pump that is overheating. The safety mechanism in the pump is turning it off when it gets too hot. This cannot be repaired. A new pump is in your future.