It could be some rust in the steerer tube which caused the fork to go out of spec.
a pipe or drain brush with some oil should clear it. You can drip some rust inhibitor, like linseed oil, into the steerer tube afterwards to prevent it from doing that again.
+1 This was my first thought also. Turn the fork upside-down, and run a round wire brush into it, and see if it helps. Once you can get the stem to fit, I agree you should oil/lube inside the steer tube to avoid future issues.