It could be very dangerous if you have infection under a crown.You should go to your dentist for examination of root canal .your dentist can repeat the procedure, although the rate of success is reduced with subsequent treatments.
If you have an infection under a crown you will need to get the tooth treated if you plan on keeping the tooth. If it is decay, you will lose the tooth if you leave it untreated. If you have an abscess, you may lose the tooth and if the infection is left untreated it could possibly lead to septicemia or death (depending on location).
You should see your dentist.