Have you tried to see if the motor will turn without installing back into the filter housing.
I have had to do this on a few occasions with mine. While it is exposed and you can tap the impeller with your finger while it is plugged in to the wall outlet. I have gotten mine to work like that. I also clean the impeller blades, the magnet and the rod it sets on and the inside of the impeller housing with Q-tips.
I then spray the inside of the impeller housing with
silicon lubricant spray. Once I got the motor and impeller to finally spin freely without having to assist it by pushing it with my finger, I reinstall into the filter. Then I put back on the tank, leave everything out of it and just run water through it giving it a change to run without having to push through the sponge and media. Then getting all back together and see if it works.
If it is leaking then you probably just need a new o-ring and can possibly find one that will work at an automotive store.
If you need to clean your sponge good in tank water and the media and just drop it back into the tank, then get you a new filter. That way the BB in the media and the sponge will survive and still help with ammonia created by your fish.