DIAGNOSIS The cup is over-tightened or vacuum-locked. Pressure from blending or thermal expansion can seal the gasket tightly against the base.
FIX Unplug the base. Hold the base firmly with one hand. With your other hand, grip the cup and press down firmly while rotating counter-clockwise. The press-down action relieves pressure on the lock. If this fails, run warm water over the outside of the cup (not the base) for 15 seconds to expand the plastic slightly, then try the press-and-twist method again.
IF THAT DIDN'T WORK Carefully use a rubber jar opener or silicone oven mitt for extra grip on the cup. Apply force only to the cup, never to the blade assembly or the base's power cord. Do not use tools like pliers on the cup, as they will crack it.
HEADS UP To prevent this, hand-tighten the cup only until you feel resistance and hear a single click. Do not apply excessive force. Always release the sip lid immediately after blending to equalize pressure before attempting to remove the cup.