If you've been sitting at your computer for a few hours now, and NOT at work, you're probably overdue to take a break and do something PHYSICAL. I sometimes find myself getting so caught up in a project I've started, that I lose my desire to go out and do anything else.
Get in the shower!!! You probably don't even know the sun is shining today! Take an hour or so to get out of the house and get some fresh (sort of) air and some sunshine on your face. It will lift your spirits, give you a burst of energy, and most likely re-INSPIRE you for when you (undoubtedly) get back on the computer.
Have a Great Day:)