Filling was used to cover up the hole left from the root canal. is that the normal procedure or should a new crown be placed?

Engineering: A post may be placed in the tooth to engineer the maintenance of strength, and then a core filling material is constructed on top. This can give adequate strength to have a crown placed on this tooth for the longer term usefulness.

Tooth build up: From your question it seems like you already have a crown and then they had to do a root canal through the crown. In that situation, just filling the hole by doing a build up is enough provided the existing crown is in good shape. However if there is no existing crown on that tooth and the root canal is done , then you need a crown on the tooth for further protection after the root canal

Crown & root canal: It depends who and when the crown was made. If dentist 'a' does a crown (ie; all zerconia or all gold) well and than the patient has a root canal shortly after, through a small opening than its acceptable to close the hole with a permanent filling and not replace the crown. Ask your dentist whats correct for your tooth.