DIAGNOSIS Hair or string has wound itself so tightly around the side brush's axle that it has locked up the motor, or the motor itself has burned out from the strain.
FIX 1) Pull off the side brush (it just pulls straight off its hexagonal post). 2) Look directly at the post it mounts to. You'll likely see a thick ring of hair and dust wound at the base. Use tweezers and small scissors to carefully cut and remove all of it. 3) Also, check inside the socket of the brush itself for debris. Clean it out, reattach, and give it a gentle spin with your hand to see if it moves freely.
IF THAT DIDN'T WORK The gearbox or motor for that side brush is probably damaged. You can test this by swapping the left and right brushes—if the problem stays on the same side of the robot, it's the motor. If the problem moves with the brush, you just need a new brush.
HEADS UP Long pet hair is the main enemy here. Check and clean these side brush axles as part of your regular weekly maintenance to prevent this motor burn-out.