Pond pumps can be used to keep the water moving and aerated for fish or to create water displays such as fountains and waterfalls. Generally, pond pumps are very reliable when looked after, but they can break down just like any other machine.
To work, the pump needs to be able to draw water in through a filter. This can become blocked with algae and other pond debris and should be cleaned or replaced.
Test that electrical power is going to the pump. If not, it's likely that either the seal around the pump has been damaged and water is entering or the cable has been damaged. Both these cases are dangerous, and the pump will need to be replaced.
The impeller can become blocked if debris has got past the filter. Once the debris is removed, the impeller should work again. However, if the impeller has been broken -- by a rock, for example -- then it will need to be replaced.