When my crown fell out, I bought some Fix-o-dent (the stuff they use to hold in dentures). It comes in a tube. Just squeeze a little bit into the crown and insert it back in your mouth. I would be very careful with what I eat for lunch. Make sure you cut the food into pieces and put it into your mouth, rather than biting into it with your front teeth. Also, take the crown out of your mouth at night. You don't want to swallow it when you are sleeping!
Oh, one more thing...take the fix-o-dent along w/you today in case the crown starts to come loose. Then you can always run into the bathroom and fix it again!