Originally Posted by
WizardOfBoz
....is it possible that your lower bearing was not fully seated on the fork bottom, and the force of impact fully seated the bearing?
Or that the lower cup wasn't seated fully? The OP didn't say what the age or make of bike is or the headset type so this is all a guess. It seems to me that an impact insufficient to even cause a pinch flat or distorted the headtube shouldn't have shortened it either.