Diagnosis
The drive coupling is disengaged or broken. The motor shaft has a plastic male coupler that mates with the blade assembly's metal socket.
How to Fix It
Unplug the unit. Remove the cup and blade. Look into the motor base. You will see a black plastic square post. If it's not protruding, it's broken. If it's there, check for debris. Use needle-nose pliers to gently pull it upward. It should spring back. If it stays up, the internal spring is gone. The base motor unit needs replacement.
If That Didn't Work
If the coupler seems intact, the issue is in the blade assembly. The metal socket might be rounded out. Compare it to a new one. If the square hole is worn round, replace the blade assembly.
Heads Up
This failure often happens from forcing the cup on at an angle. Always align the blade assembly squarely before pressing down and twisting.