yeah, just ask for it back. he'll either give it back or tell you what happened to it. If he's a decent human being and it was stolen while under his care, he will get you another bike or somehow compensate you.
but don't count on it.
think positive, your bike might be OK.... he might have lent it to someone else! in which case you have more chasing down to do... boy, that kind of sucks.
"Think of bicycles as rideable art that can just about save the world". ~Grant Petersen
Cyclists fare best when they recognize that there are times when acting vehicularly is not the best practice, and are flexible enough to do what is necessary as the situation warrants.--Me