DIAGNOSIS Hair and threads are wound tightly around the side brush axle, jamming it.
FIX Pull the side brush straight off. Use tweezers or small scissors to cut and remove all debris wrapped around the metal post underneath. Reinstall the brush, making sure it clicks into place.
IF THAT DIDN'T WORK The side brush motor module might have failed, but it's rare. Before assuming that, try a brand new, genuine iRobot side brush. Knock-off brushes sometimes don't seat properly and won't engage.
HEADS UP Check and clean this every 2-3 cleans if you have pets or long hair. Letting it get fully wound can burn out the motor.