You have a longstanding infection. The bacteria that cause the tooth to break down continued to destroy your tooth after the filling was lost. Eventually, bacteria killed the nerve of the tooth. You don't feel anything because the nerve in your tooth is dead.
Root canals should not be painful. Numbing medicine is used to prevent pain during the procedure.
You need to get the infection cleaned out of your tooth. Otherwise the infection could spread into the soft tissue. At that stage you will develop a large swelling, which could lead to hospitalization.
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