Dental help: Crown fell off molar root canal.....8 years ago. (Photo)

At this point, there is no way to save the tooth, so when you plan to do SOMETHING, it will start with extracting whatever remains. The choice at that point would be to bridge or place an implant. At your age, the best investment would be the implant. 

Based off the picture alone, it appears the the roots of the molar are separated from each other and there is gingiva (gum tissue) growing between them.  If that is the case the only options is to have the tooth removed.  From there there are several options that may be available for replacing the lost tooth.  It would be best to be seen right away for evaluation because the longer you put this off the more risk you are of infection.  The infection could destroy the bone around the tooth (and possibly teeth surrounding) and cause replacement to be more difficult and expensive.  Replacement options (depending on the condition of the area) are implant, bridge, partial denture (different designs).