There are two conditions that can keep me from writing anything: the flu and large amounts of stress. Now, most freelance writing veterans will tell you to muscle through and write no matter what. I believe that sometimes you need to stop and learn your limits.
I used to be part of the man-up-and-do-your-job tribe. That was until I wrote an article while I had the flu. I was on a deadline and my article needed to be turned in within the next few days. So, I pumped myself full of flu medicine and wrote my little heart out. I edited the article and sent it on its way. A few hours later my editor sent me an email that pointed out all of the embarrassing typos.
Yah, not gonna repeat that!
Do you have a time when you refuse to write?