One of my friends once told me that "Writer's block is the soul's way of saying you're not ready to write about it". This may be, as royalladyemma says, because you're not longer enthralled by whar you're writing, or don't connect with it anymore. Maybe take a few moments and bang out something completely different - this may have no sway on you, but when I started writing drabbles over tw100 it really reinvigorated my writing, because it was something completely new, in a completely different style. It was exciting and new and I found myself enjoying writing again.

After this, my other fics seemed to flow naturally again. They just needed a shot in the arm, something different, for all the synapses to be re-energised.