I got a big-toe nail like that once - once at school I took the wrong trainers for P.E. (somehow), which were too small but I wore them anyway. During the lesson I kicked a wall by accident (was playing basketball indoors and trying to avoid someone I think), right on the big toe. It went all purple and died, but stayed attached. So I had to wait for the "new" nail to grow before it dropped off, something like four months later.