Diagnosis
A vacuum seal has formed. Heating the air inside the jar and then cooling (by blending) creates lower pressure inside, suctioning the lid on.
How to Fix It
Do not pry or use tools. Run warm (not hot) water over the lid and the top rim of the jar for 20-30 seconds. This warms the plastic and equalizes pressure. It should twist off easily.
If That Didn't Work
Carefully insert a thin, blunt tool like a butter knife tip under the edge of the vented measuring cap (the small center lid). Gently lift to break the seal. You should hear a hiss. Then twist the main lid.
Heads Up
Never blend piping hot liquids. Let soups cool slightly before blending. The steam and rapid cooling are the main causes. Forcing the lid can crack the jar rim or the lid's locking tabs, making it unsafe.