DIAGNOSIS You've got a vacuum lock. As the canner cools, a strong vacuum can form inside, pulling the lid down so tightly that the locking lugs bind against the pot's rim. It's frustrating but a sign of a good seal.
FIX Don't force it. 1) First, double-check that the pressure gauge reads zero and that the overpressure plug has sunk back in. 2) Ensure the little metal safety valve stem in the lid is down (not popped up). 3) Try gently tapping the lid handle with the handle of a butter knife or similar to jostle the lugs. 4) If it still won't budge, put the empty canner back on a cold burner. Turn the heat to the lowest setting for no more than 60 seconds. This will slightly warm and expand the metal, often breaking the vacuum seal. Immediately try to turn the handle again.
IF THAT DIDN'T WORK Look at the gap between the lid and the pot. If you see it's slightly uneven, the lid might be cocked. Have a helper hold the pot steady while you firmly grip the lid handle and try to turn it counter-clockwise, applying steady, even pressure.
HEADS UP Never use tools like pliers or wrenches on the handle—you'll break it. If the mild heat method doesn't work, just walk away for another 30 minutes. The pressure will almost always equalize completely given a bit more time.