x-men canon question

For those of you who know more about X-Men than I do -- how long has Xavier been in that wheelchair? Was he already crippled when Jean was his student, or did it happen later? And was Magneto responsible, or am I just making that up?

It's for my xmmficathon fic, so while I'm cool with just making up a backstory that fits my fic, I'd rather go with canon if I can...