Why Would an Old Root Canal Start Causing Pain?

Hi:
There might be infection that is not showing up on the X-ray yet.  Sometimes infection takes up to 3-6 months to show up on an X-ray.  Another reason could be cracked tooth.
Best,
Dr. Maddahi

Teeth that have already been treated with root canal therapy can at times become reinfected due to a variety of reasons. One of the more common causes is simply time, with time the materials used to seal the canal system can break down and fail, creating a potential space in which bacteria may possibly enter again and re-colonize leading to a re-occuring infection of the same tooth. Regardless of the reason of re-occuring infection or pain, if the tooth is mostly intact and has a good prognosis of further survival, re-treatment of the tooth with another root canal therapy is indicated. 
Keep in mind whenever re-treatment is performed, the prognosis is poorer  for the tooth the second time around. As such, careful consideration should be made on whether to re-treat an already root canal treated tooth or perhaps consider completely replacing the tooth if the prognostic factors and chance of success of a re-treatment seem to be low. Only your doctor can make this assessment and you should speak about all your options with your treating doctor. Good luck!

Thanks for the great question. There are several possible reasons for an old root canaled tooth to start hurting. Most likely reason a leakage between outside restoration covering access to the canal. Nothing lasts forever. That's the reason why 2/year dental check ups are so important. Prevention is a key to success and longevity. Another reason could be a fracture of the root, which actually comes down to the first reason-leakage. It also could be adjacent teeth that hurt but feels like it is one with a root canal. Clinical examination with an experienced doctor will help to diagnose the issue properly.
Best of luck to you.