There are several things you can do to reduce pain from a broken or lost filling if you can't get to the dentist straight away.
If your tooth is very sensitive, try the following:
Apply clove oil, or an over-the-counter dental anesthetic like Orajel (benzocaine) or Anbesol (lidocaine), all of which you can find at most pharmacies.
Avoid foods and drinks that are very hot or cold.
Use an over-the-counter pain medication like ibuprofen or acetaminophen.