When feeling exhausted, tired, sick, generally used up or lethargic, it is time to fuel up. That fuel is in the form of reviewing your visions for your future self in the categories you have for yourself. Then recommitting to living the life you want for yourself. Simply getting back in touch with your personal 'Why' will refire the engine. But don't just refire and remain idle, when you look at those visions, realize that they don't happen all by themselves it is still 'you vs the work', follow your vision down to the level of your goals, then the projects or systems you've created to reach into those goals and grab a task you can do right away.
My recent experience with this was after a long weekend project and back to back busy weeks. I just felt spent. All I wanted to do was lay on the couch, eat delicious (but not good for you) food, and drink beer. I had a realization that if I do those things I will not feel better but actually worse. Then I remembered that my meditation and exercise are the foundation of my energy. After completing a workout I was able to feel alive again and have the energy and attitude to have a fun day with my family.
MetaphorX4 - you've just run out of gas and your rig is stuck until you refuel. Filling up that tank requires you to be very intentional, you must find a source of fuel, and bring it to your 4X4. Filling the tank is only step one, you have to reignite the engine, and then hit the gas to get going again. Our fuel is our commitment to our visions, our tasks that come from our goals and systems we created is the work that burns up that fuel and turns it into momentum that sends us down the trail. Better to realize these things and create a daily routine of reviewing your visions, goals, and tasks - recommitting yourself to creating the life you want - daily to keep your tank full.