The tooth's nerve root should be dead after my root canal 6 months ago. so, how did i get a dental abcess if all the canals were perfectly done?

It happens: Sometimes an extra or inaccessible canal can cause the situation you describe, even years after the root canal procedure. It will likely require another look to search for one of these or possibly an apicoectomy to remove the very tip of the root where unfavorable anatomy sometimes makes a perfect fill almost impossible. Best to get back to the treating dentist for an assessment.

Secondary infection: If you have another infection as you suggest there are a few explanations. You may have a fracture allowing in bacteria. You may have the original infection acting up again. Your infection may have an origin inside or outside the roots. Your dentist or endodontist should be able to diagnose your particular problem.

Depends: There may be some residual bacteria that remained alive after the root canal system was sealed. This can be resolved with antibiotics. Alternatively, there may be another canal that wasn't seen or treated by the dentist that performed the rct procedure. Return to your dentist for an evaluation.

see dentist: Could be if retreat of root canal needed because of missed canal or canals not filled completely..Gum infection can also cause periodontal abscess..Get it checked by your dentist..