Theirs different versions of what happened from my experience in one of the older ones they said he got it from war The Ultimate X-men version said or at least hinted to Magneto did it The Marvel directory says "Against an alien calling himself Lucifer, the advance scout for an invasion por his race. In retaliation, Lucifer dropped a huge stone block on Xavier, crippling his legs."
It depends on which time you're talking about, because in just the 616 universe (the normal comic mainstream universe) he's been in and out of being paralyzed 7+ times. No joke. He gets his legs and loses them again. It's an ongoing cycle. In other universes, there's other stories as to what happened.
