How is a second root canal possible, if there is no root and no pain?

Root canals work 90% of the time, and root canals done today with better techniques even exceed that success rate! It is possible that all the bacteria weren't removed before the root canal filling done. Also possible is that the root canal filling, although removing the entire nerve, did not go down all the way to the root tip, leaving a very small area that could harbor bacteria and continue to infect the bone. And sometimes there are extra canals in the root that weren't detected and filled.Antibiotics don't work because it takes a good blood supply to an area to work, and there is none inside the root canaled tooth. Best thing is to go to a root canal specialist, he will remove the old root canal filling, redo it and give you a better chance of saving the tooth. Sometimes it is necessary to do a small gum surgery to clean out the infection. All that the endodontist (root canal specialist) can explain to you at your consultation.

If the tooth is worth saving, then a re-treatment could be indicated.I would seek the opinion of an Endodontist (specialist in root canals) and
find out the prognosis.

When a crown loosens, it is possible for bacteria to re-enter the canal and re-infect the area. Although you had Root Canal done on that tooth many years ago, the "root" is still there. It was the "canal" that runs throught the root that was treated by root canal therapy many years ago, and now has to be re-treated. Antibiotics will temporarily mask the symptoms, but will not cure your problem.