Heat the pan again and quickly remove the lid. If you have to, put a cold, wet cloth on the lid. The heat should expand the bottom (the pan) and the lid should lift out. Don't leave it while the pan is heating. If you wait for the lid to get hot, you'll still have the same problem. Start testing the lift as soon as you start heating the pan and continue until it comes off.
Sometimes a very thin paring knife tip will release the pressure inside. You could also put it in the freezer. Things tend to shrink up in there and the lid, being aluminum will freeze faster than the cast iron. Good luck!
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