If the crown came off and the tooth is intact, you can use a number of over the counter options to temporarily fix the tooth until you can get into your dentist to have it permanently cemented.-White tooth paste (the gel stuff just doesn't stick as well)-Temporary dental cement (you can find this in any pharmacy near the other dental products)-Fixadent or polygrip (buy the gel not the powder)
visit dentist as soon as possible to cement that cap . do not use any type of glue, otherwise u will worsen the condition.