A very constructive guest post at Charlie's blog discusses some of the most common causes of writer's block (and it very often does have mundane causes) and how to address them.
The opening line, a long neglected glasses prescription that is causing non-specific symptoms that is wrecking havoc with your life, is something that actually happened to me two or three years after graduating from law school and then moving to Colorado. I had been having awful headaches for months, a number of inexplicable close calls while driving, and just generally felt awful.
I went home for Thanksgiving, stopped by my usual childhood optometrist, because I hadn't been for a while and wanted a new style of glasses for the new Western Colorado me, and voila, all was explained. My prescription was much, much worse than five years earlier when I'd gotten my last pair of glasses towards the end of my college career. And, in just a few days, all of the symptoms that I hadn't know that I'd been suffering from were gone.
Sometimes problems with very complex effects have very simple causes.