Is it too late for a crown following a root canal on a molar 1.5 years ago?

Usually a filling falls out if there is decay.  If the tooth had been treated with a root canal treatment awhile ago, and the filling has fallen out, chances are bacteria has leached into the root canal seal.  If one still wants to save this tooth, the dentist needs to make sure all the decay is removed and the root canal procedure retreated, provided that the tooth has enough tooth structure left for a post or filling and a crown.
More than likely the tooth will need to be removed.

The best solution is first to find an experienced General dentist who offers comprehensive care and routinely provides Endodontic (root canal) treatment of molars as well are Crowns and other restorative care and to do this as soon as possible.  His evaluation is critical, but from what you are stating and from my experience over 30 years of treating 100's of teeth simiIar to what you are describing  I would expect that what is needed is: 1. to remove any debris from inside the tooth where the root canals were treated and make sure the remaining tooth structure is sound. 2. to make sure the root canal treatment was done correctly and that the roots are still sealed. 3. to build up the missing tooth structure internally including where there was any previous fillings/restorative care (this may entail using a post and core using the canals to anchor and strengthen the build-up), and 4. placing a well fitting and properly contoured crown. My experience has been that if this is done properly your tooth can last for the rest of your life.