Check that the line circuit breaker is not off, and ensure the fuse has not blown and is not loose. These are common causes of the pump failing to run.
Similarly, check that the float is not stuck and the water level in the sump is not too low to activate operation.
Ensure the arrow on the check valve points in direction of flow. Otherwise, it is installed backwards -- the most common cause of the pump running but failing to deliver water.
If this is your problem, check that the discharge shutoff valve is not closed. And fix any air locking that has taken place, by unplugging the power cord and plugging it in again several times.
Remove the pump and clean out openings, if the fuse blows or the circuit breaker trips when the pump starts. This is indicative of a clogged inlet in the pump base and should be remedied by this simple cleaning process.
Clean the pump impeller, if the motor is inexplicably ceasing to run. This may be due to clogs, and the cleaning process should fix the problem.