A lost filling or crown can be painful because the exposed tooth tissue is often sensitive. Especially to temperature, pressure or air. Put the lost crown in a safe place and make an appointment to see your dentist soon. The tooth could be damaged more or move into the space where the crown was and your crown may no longer fit.
You may also apply a little clove oil to the sensitive tooth with a cotton swab for pain relief.  Clean the inside of the crown. Then coat the inside of the crown with tooth “cement,” (available at your pharmacy) then slip it back over the tooth.  You can also use denture adhesive or petroleum jelly. Do not use any household glues. They are not safe to put in your mouth.
If you can’t find the filling or crown, you can put dental cement directly on the tooth surface.