Sharp pain in the tooth is because of decay in tooth in which the decay is so deep that it is causing the pulp exposure. In initial stages of decay there is no pain but when it goes deep because of nerve exposure there is shooting pain. sometimes decay is not visible as it is at proximal surfaces of tooth which are not visible to naked eye. Now for this you have to go to dentist who will do clinical examinations and do xrays to make diagnosis. If is due to deep decay then you have to undergo Root Canal Treatment followed by crown on tooth. for more info on RCT you can read this article... http://www.identalhub.com/article_why-is-root-canal-treatment-rct-required-299.aspx