Should I buy over the counter cement to put back the crown and post until my doctor appointment about 20 days from now?

If your crown came off do yourself a favor and see a dentist to evaluate, find out the reason why the crown came off and re-cement your crown properly. If you do it yourself, you may cause gum infection, bit problems, and not placing the crown the right way, and possibly fracture the root, if you have a post and crown. 

I would not recommend attempting to temporarily cement the post and crown. The best advice I can give you is to see your dentist right away to address the issue. If the tooth is in good condition with no decay present, the post and crown can be recemented. The cost to have this done is minimal, even without dental insurance. If your dentist finds decay or a fracture present and the crown can not be recemented, I would recommend discussing your options (possibly a new post and crown, extraction and implant, extraction and bridge). At that point, you can determine whether or not it'd be beneficial to wait until your insurance is effective to have treatment completed. I hope this helps.  Follow me for more questions and answers.

Hi:I definitely suggest for you to go to a dentist right away to make sure the crown and post is properly recemented.  Improper cementation of the tooth can result in bite issues and further damage to your tooth and crown.Best,Dr. Maddahi

Dear vnahi54:

No you should not.  Before you attempt to do anything, see if you can seat the crown and post properly all the way. If you can,  please watch the video I have attached on how to do it. If you can not, you need to call your dentist and tell the staff that it is an emergency, especially if it is a front tooth.   Decide for yourself if you can walk around without it. There are plenty of dentists around who would be more than happy to see you right away.