The problem with photography (or really, any creative pursuit) is that you can have all the variables in place and have prepared yourself as well as possible, only to feel the creativity drift away from you like some nebulous cloud you grasp at only to watch your hand pass right through it. This video examines the idea of when it's time to take a break.
Coming to you from Thomas Heaton, this helpful video follows him as he deals with a day when he just can't seem to accomplish what he wants. Photography, like most anything else, rewards persistence, but of course, there's a difference between fruitful persistence and simply frustrating yourself; the trouble, of course, lies in having a feel for when you've crossed that line and when you need to give your mind and body a rest so you can try again with renewed vigor. That line differs for us all, but I think it's important to know for ourselves where it is. As Heaton points out, creating images is very much an experience, and our enjoyment of any such thing has intimate ties to our ability to create within that moment. Be sure to take care of that part of yourself just like you would take care of your equipment. As you can see in the video, it'll reward you when you try again.