Dogue de Bordeauxs

ok someone please help, just got home and let my girls out lab and bordeaux. both seems fine pet their bellies, we go out front to potty and lab runs down to mailbox to check p-mail and i turn around my bordeaux locks up stiff and falls over and hits her head on the way down and really doesnt move at all, i call my neighbor to come over and help open the door so i can get her in, i lay her on the couch as shortly she is back to normal......i need help on what happen.