"Investigation not followed through"

In summer last year, I received a letter basically telling me that the Dr's team could not find the cause of the numbness in my feet. After this I have not heard anything more, not even to ask if the symptoms are still persisting, if there had been any change be it an improvement or deterioration, nothing.

The problem persists, it affects my mobility, and I am left without any sense of closure. What am I supposed to do? How do I get to the bottom of the problem I am experiencing? I should not have been left feeling abandoned in the way that I do.