Can root regrow after having root canal therapy?

No: Any tooth structure removed during root canal therapy will not regrow. However, only a small amount of tooth structure is removed through the center of the roots. So is the pulp which is also in the canal area. The root as a whole is still functional after it is filled.

No: Root canal therapy removes the nerve and organic matter from inside a tooth's root canal system and then fills it with a material (gutta percha) and sealer. The root remains after root canal therapy, it's just been sterilized and filled. Sometimes, bacteria can invade this system and cause the root canal therapy to fail and need re-treatment.

Regrow: the only exception to this is when adult teeth first come ,especially in the front of the mouth, due to trauma( primarily) or decay the nerve needs to be treated , in procedures called apexogenesis or apexification, in order to maintain the tooth. The goal isnt to regrow nerve tissue, just finish off the growth of the hard tissue