I'm FIREd for almost 5 years now, and I have days like that (total inertia) sometimes. It gets worse during the winter.
The way I deal with inertia is to do some small project that requires light activity and takes an hour or less, such as laundry, dishwasher fill/empty, repot or trim dead leaves off houseplants, reorganize a closet, go through my file cabinet weeding out old paperwork, prune grapevines or shovel snow, wash curtains, etc etc.
I put on lively music CDs or digital cable music channels and get up out of the chair. It improves my mood and I get a feeling of accomplishment.
Make yourself a list and when you feel that major inertia, pick a project from the list and get it done.