DIAGNOSIS The motor bearings are failing or the drive gears are stripping. The burning smell is insulation on the motor windings overheating.
FIX Stop using it immediately and unplug it. This is a critical failure. Do not attempt to run it again. The motor unit needs professional repair or replacement.
IF THAT DIDN'T WORK There is no alternative fix. Continued use will destroy the motor completely and is a fire risk.
HEADS UP This often results from pushing the blender too hard against the bottom of a pot or jar, creating extreme lateral force on the shaft and bearings. Let the blade do the work.