I don't think I get it. There are times when I can't write, but that's just because I've spent too long at the computer and need a break. And there are other times when I'm so riddled with doubt (a common failing in an author!) that I find it hard to put words on the page, but the cure for that is to just write and not worry too much about what I'm writing and whether it's any good. So I guess the answer is - throw some words on the page, any words, and keep going until it gets easier.