Brush no turning belt is good

If you're sure the belt is good, them you'll have to open it up. Make sure there isn't anything stopping the brush, like a bunch of hair or whatnot. Check the full-length of the brush. Take it out and make anything like that is cleaned off. Then put it back in without the belt and see if it will spin. If yes, then reinstall belt and test. If no, then there could be an issue inside the brush. Every vacuum I've worked on has 2 bits that slide into the vacuum cleaner on the ends and holds it. Alot of the time those pieces remain stationary, and the brush spins on bearings. Might be a problem with that. If everything checks out and it still won't spin then you'll need to check the motor.