Crowns can come loose or fall off. You’ll want to get to the dentist as soon as possible to have it reapplied or replaced because a crownless tooth can decay quickly. If your crown comes off, sterilize it with hydrogen peroxide and place it in a baggie so you can bring it with you to your dental appointment. Don’t eat on the side where the crown is missing.
If it will be a day or two before you can get to the dentist, you can try reapplying the crown by using a temporary crown cement (available in drugstores). But, carefully clean the tooth and crown first before attempting to reapply. And, make sure you place the crown in the correct position on the tooth.