DIAGNOSIS Hair and thread are wrapped tightly around the brush roll ends, jamming the bearings. The motor disengages on carpet to prevent damage.
FIX Turn the vacuum off. Press the brush roll release lever and remove the cleaner head. Use scissors to cut and remove all debris wrapped around the ends of the roller, especially deep in the bearing housings. Pull the debris out, do not yank. Reinstall the head until it clicks.
IF THAT DIDN'T WORK The brush roll motor or the drive belt inside the cleaner head has failed. You'll need to replace the entire cleaner head assembly (part VCA-T75A).
HEADS UP Check and clean the brush roll weekly if you have pets or long hair. Letting it get fully jammed can burn out the motor.