The impeller is a magnet, and is a bit "stuck" to the motor magnet so might not turn freely. Simplest way to see if it is running is to submerge the filter (disassemble enough so the impeller is visible) and plug it in briefly to see if the impeller turns. <This will make quite a splash - so do it in the tub or something.>
If the impeller is not turning, you will need to pull it & clean it. There is usually a little cover/retaining ring over the impeller. Once you remove that <the blades of the impeller will then be completely free>, you can pull straight up & out. The magnet will exert a bit of force so you will need to pull, but you should not have to force anything.
Check that there is no sand or other deposits in the impeller housing/ magnet. With hard water, there is often a film of hard water scale. You'll need to soak that off with vinegar (or CLR lime remover if it is really bad.) Clean & reassemble. It is recommended that you clean the impeller monthly in areas with hard water to prevent it seizing.
With the impeller out, check that it is not broken. <The magnet should engage the blades with 1/4 turn.> You'll need a new one if it is. <Happened to me this WE with my pump!