I spent all of today completely burned out. I had no kind of physical energy and no mental energy either. I also happened to be pretty useless, too. I stared at the computer at work today and just didn't know what to do. I had 5 different projects to work on and could never remember any of the things I was supposed to do.
It was so bizarre feeling completely useless. No matter what I wanted to do, I just couldn't do it because I was completely out of energy.
Everything needs energy, even thinking. If you are out of fuel, you really won't be able to get anything done. If you're spinning your wheels and going nowhere, take a break, get some fuel (rest, food, water, etc.), and try again later.