It depends on the process. When I wrote without an outline, organically I guess, there were times I couldn't face even the idea of sitting down and working. The bad part is that I have the itch to keep creating, it can make one pace around and mutter which makes your coworkers and/or family quite nervous. So I just ignored the itch and did something else. I read, or played games, or did something physical like walking or biking (I really recommend a kayak and a nice quiet river). Occupy your mind with something else. It needs a rest sometimes.
The other writing process is to use an outline, chart the trajectory of the book, give it the bones before trying to add characters and plots. It's rarely that simple though. Sometimes I want to think about characters or situations instead. That's okay, I just know not to try to add them just yet. My writers block using this method is coming up with what happens next in the plot. The quick answer is "don't worry about it," skip it and leave a note, work on what happens next. Most times I come back to my skips and it's pretty obvious what happened. If not, I know I might be looking at a plot hole or corner.