When I have days like this, I try to work on bugs. That way I don't have start anything, it's already been written. I just have to fix it. For me it's pretty easy to get motivated to fix a bug because I can just step lazily through the debugger until my momentum catches up.

If I don't have bugs, I'll usually work on a Kata for 15 min or so. Something I've written hundreds of times and can do from memory. This helps my momentum get caught up, too.

I prefer working on bugs, though. I feel more productive.