The pond pump uses an impeller, a small reverse propeller, to take in water. If a pump burns out, most of the time, it is the impeller that gets clogged, makes the pump work too hard and burns it up. Most pumps have a safety mechanism built into them that shuts the pump off when it gets too hot and lets it run again when it cools off. So if you notice your pump working intemittently, know you have a problem. UNPLUG the pump and check your impeller immediately. It is usually on the bottom behind a screen that you can easily detach. Chances are you will find gunk in there. It may be too late to save the pump, but it is worth a try. Wash the pump with a strong hose stream from the outgo end. That will help get the junk loose. If you can't get to the impeller with your hand, use a screwdriver to start it moving again, much like you use a broom handle in your garbage disposal.
To keep junk out of the impeller, protect it with a pump prefilter and clean it often.
Most pumps come with a prefilter, often a small cylinder filled with foam rubber that screws on the impeller end of the pump, other times, the pump will arrive already in a box of foam rubber. Pondmaster pumps, a favorite of mine, arrive complete with a black box and layers of prefilter material.