I once had a filter that this happened with every time it was switched off during a water change.
Do you mean an internal filter? There are a few things to check.
First, with the filter unplugged, take the pump part off the media part and have a look at the impeller. You may need to look at the instructions to find out how to get in. Are the impeller and the impeller well both nice and clean? If they are not, clean them. You can use a cotton bud to get right down into the impeller well but be careful the end doesn't come off inside the well. You'd then need to search for some tweezers to get it out. (done that!)
If that doesn't work, the next thing to try is a bit fiddly and can result in an unexpected shower. Remove the cover of the impeller well and hold the motor part under water with the outlet pointing downwards. Plug it back in to the mains - easier with a second person. Then flick the impeller to see if that will get it to start. If it does, good; now put the cover back on and refit the motor to the media part of the filter, keeping the motor under water all the time and pointing into the tank.
Hopefully you will now have a functioning filter and are still dry.
This is what I had to do with that filter of mine at every water change.
