Hi Mark
You really ought to see a dentist for a proper opinion, diagnosis and treatment plan. I appreciate that you are anxious of dental treatment, but if you explain this to your dentist I'm sure that they will be sympathetic.
From what you have described, it sounds as though you have had a partial root canal treatment, and then subsequently the crown of the tooth has decayed and fractured, leaving the roots behind.
Assuming that the tooth is 'non-restorable' (ie beyond saving), you really should consider having the roots extracted. Although non-symptomatic at present, it is only a matter of time before bacteria that have already invaded the root canal, progress through the apex of the tooth and form an acute periapical abscess.
I can sympathise that the 'easy' option is just to leave alone at present, but unfortunately the situation will only deteriorate. Not just in terms of risk of infection with the associated pain and swelling, but the further the root decays, the potentially more difficult it could be to extract.
Hope this is of help, and that you are able to get the problem dealt with without too much anxiety.