The most important thing to do is to follow excellent oral hygiene! Make sure to brush and floss your teeth as well as you can. Yes, even over the prepared (cut down) tooth. You want to make sure that you do not accumulate plaque on the tooth that has the missing veneer. If you do not clean the tooth, your gums will get inflamed and proper cementation will be very difficult.
The best thing to do is to try and locate a dentist that can re-bond the veneer for you. However, if you do not want to do that, or you can wait for your dentist until after the weekend, you can try using Fixodent. It is an adhesive for dentures and it is available at the pharmacy without a prescription. It is a sticky, pink gooey material that can fix the veneer in place. Do not buy temporary cement and try to cement the veneer by yourself. It is not the proper material to use and you might destroy the veneer. The Fixodent works fine for most emergencies such as yours. If the veneer does not stay, then it is OK to leave the tooth uncovered. As long as you are not sensitive, and/or are in pain, nothing will happen to the tooth over a short period of time. As I said, just make sure that your oral hygiene is impeccable
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