DIAGNOSIS: Liquid or food residue is seeping past the blade assembly seal and into the drive socket on the motor base. This is super common if you overfill the cup or blend hot liquids.
FIX: Unplug it. Wipe the exposed silver drive socket on the base with a damp cloth or paper towel. Do not submerge the base or run water directly into it. To prevent it, never fill the cup past the MAX line, especially with runny ingredients. After blending, always twist the cup off the base before you unscrew the blade assembly from the cup over the sink.
IF THAT DIDN'T WORK: If a significant amount has gotten in and dried, you might hear a gritty sound when it runs. You can try using a cotton swab slightly dampened with water or vinegar to gently clean the grooves of the silver socket.
HEADS UP: Getting liquid into the drive socket is the main way to kill the motor. It's not officially user-serviceable. If it stops working after a leak, the motor windings are likely corroded.