Has it been slowing down a lot lately or was it working fine and then just stopped?

Check all connections. Make sure you have power where you're supposed to and a good clean ground. Make sure the switch is clean and isn't worn out.

Failing all that, removal, disassembly and cleaning/repairing of the motor is required. You'd be amazed at how simple it really is. I did it when I was a stupid kid in Grade 11.