DIAGNOSIS The motor is seized or the blade assembly is mechanically locked. This is almost always a failed drive socket or a seized blade bearing.
FIX Unplug the blender. Remove the container. Look at the black plastic drive socket in the base. If it's cracked or the square hole is rounded out, it's stripped. Order a new drive socket (part # 14497) and a 3/16" hex key. Remove the four screws on the base plate. Replace the socket. Reassemble.
IF THAT DIDN'T WORK The bearing in the blade assembly is seized. You need a new container. The blade unit is not user-serviceable.
HEADS UP Never run the motor dry for more than a second. That heat can damage the windings. Always have the container locked on with liquid in it before testing.