Diagnosis
The blade assembly's metal drive socket has stripped the plastic splines inside the cup's base. This is a wear failure.
How to Fix It
You need to separate the pieces to assess damage. Hold the cup base firmly. Use pliers to grip the metal ring on the blade assembly. Turn counterclockwise with steady force. Do not twist the cup. Once off, inspect the plastic splines inside the cup base. If they are rounded off, the cup is trash.
If That Didn't Work
If the assembly is truly fused, the threads may be cross-threaded. Submerge just the stuck assembly in hot water for 60 seconds. The thermal expansion can break the seal. Try the plier method again.
Heads Up
Never run the blender without the cup fully seated and locked. The vibration accelerates spline wear. Hand-tighten only; the motor provides the final torque.