I think everyone's just as clueless at the moment. All I've heard is that she was on a beginner's course, she wasn't wearing a helmet, and fell--but it wasn't a hard fall or anything. Then her headaches got worse, and then her instructor told her to go see a doctor.
I do hope she gets better though (And I hope that brain dead fact isn't true, but I've heard about it in a lot of places). I feel sorry for Liam Neeson and her family. (What's even more ironic is that I'm watching Nell in one of my classes. I feel terrible when watching it now.)