Thanks for putting up your question on Just Answer.
Assuming your tooth was not root canaled before the crown was first made, there are two possibilities why there is pain:
1. The tooth has become tender due to an inflammed pulp (pulpitis), and now needs a root canal. The pulptitis could be because of the thermal insults that it received when there was no crown covering it (in the period that it fell off).
2. The crown has been recemented improperly, and is impinging on the gums.
For now, take anti inflammatory medication. Switch between taking 400 milligrams of ibuprofen (e.g., 2 tablets of Advil), and 1000 milligrams of acetaminophen (e.g., 2 tablets of extra-strength Tylenol) every 3 hours. Be careful not to use these medications if you have had previous adverse reactions to either, or if you have had gastritis or peptic ulcer disease in the past, or if you have liver disease.
Get an appointment with your regular dentist to get the tooth checked out as well as your bite (due to an improperly fitted crown).
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Good Luck !