Cub Cadet RZT 50 won't move forward or backward on both sides

I was mowing when my Cub Cadet RZT 50 stopped moving. It would not move forward or backwards on both sides. I checked the drive belt. It is worn but not broken. The idler pulley is putting pressure on the belt. Both arms appear to be working correctly. The drive belt appears to not be moving at all. I had to disengage the hydrostatic drive to move the mower at all.

Any thoughts?

Thanks

If the engine pulley is turning and the belt is not.
The belt is worn out replace it.