Diagnosis
The drive coupling on the motor base is stripped. The plastic square that engages the jar's blade assembly has worn down and can't transfer torque.
How to Fix It
Unplug immediately. Look at the black plastic square drive socket in the center of the motor base. If the corners are rounded off, the coupling is stripped. This requires replacing the entire motor base drive assembly. It is not a user-serviceable part.
If That Didn't Work
If the drive coupling looks fine, the issue is in the jar. The blade assembly's square drive may be stripped. Unscrew it and check. Replace the blade assembly if damaged.
Heads Up
This is often caused by over-tightening the jar onto the base or trying to blend frozen items without enough liquid. Never force the jar onto the base. The humming means the motor is trying to run—let it burn out and the repair is much more expensive.