Most probably it's ulcer you are talking about. It usually happens after certain dental procedures. May be sharp tooth which has damaged the soft tissue or you might be allergic to temporary fillings which is given during the treatment. You better consult the same dentist where u undergone root canal treatment. He will advise you some medications and gel.

Madam your question I didn't understand whether you are having hole in the or teeth if hole is there in the teeth u can fill it with dental cement