DIAGNOSIS The motor is overheating or the insulation on internal wiring is failing. This is a serious safety hazard.
FIX Unplug the blender immediately. Do not use it again until repaired. The cause is often a failing motor or a jammed blade assembly causing excessive load. Check for a seized coupler as in the first fix.
IF THAT DIDN'T WORK If the coupler is fine, the motor windings are likely damaged. This is not a user-serviceable part. The unit should be replaced.
HEADS UP A burning smell is a critical warning. Continuing to use it risks electrical fire. Discontinue use.