I've had some pretty bad accidents (car, skiiing, bmx) -- and I've always been able to piece things together.

If you don't find a road imperfection, I'm going to go along with what dvdslw said -- some kind of blackout lead to the loss of control.

Either way, you got better stuff to worry about right now, like how to get around, how to shower, how to sit down to poop...that stuff all sucks really bad when you're hobbled up. Just rest for now man and stay in touch with your doctor. Eat as much as you can -- your body is going to need the extra protein to repair.