I'm sorry... I have no idea, but see the same problem coming in the next month or so with my LO. He now turns onto his tummy in his crib at night, wakes up and is very upset. During the day, he can roll back over to his back, no problem. But at night in his crib, he seems to forget how. Good luck. I'll watch this thread to see what good advice you get!
Hang in there!