As far as the overall course of wrestling, there are two correct answers:
Originally Posted by Jupiter Jack Daniels
Magnum TA's car accident in 1986.
Originally Posted by Matthew Castillo
Shawn Micheals losing his smile ahead of Wrestlemania 13, which lead to the Hart/Austin Double turn.
Magnum's career-ending injuries singlehandedly, in my mind, changed the course of Crockett, which eventually leads to WCW, the Monday Night Wars, et cetera et cetera. Its importance cannot be overstated.
The same goes for Michaels' (ironically, likely fake) "injury" in early 1997, which leads to Austin's fantastically successful face turn, which leads to WWE turning its business around, and off we go.
I actually don't think Austin's neck injury is nearly as important as those two, because I don't think much changes as far as wrestling's overall history. If Owen doesn't injure Austin, Austin continues on and wins the title anyway, WWE wins the war anyway, WCW disappears anyway, and eventually Austin retires since he was battling a bunch of other injuries anyway, and we end up roughly in the same place.