Pain in molar that has an old root canal and old crown, what could the problem be?

Be aware root canals can fail ,you definitely need to see your dentist to take x-ray and evaluate the tooth, they may place you on antibiotics and the pain goes away for  some time but worst case is because most of pain on old root canals means you may have a cracked tooth in that case the tooth not savable and end up in extraction

Hi:
Sorry to hear about the pain.  There is a possiblity of infection or cracked tooth.  It's best to have your dentist take an xray and examine the tooth for a definitive diagnosis.
Best,
Dr. Maddahi