You need to see a very experienced general dentist or a root canal specialist (endodontist) if your root canal treatment fails. They can try to do the treatment over again, which requires the removal of all old filling matter, posts crowns, and anything else in the pulp space. They can then disinfect the area and seal it.
Once the re-treatment is finished, you can see a restorative dentist to have a crown fitted. This crown will protect from fractures to the tooth in the future.