When we could write and we just don't want to. Personally, I think both are valid, depending on several factors. Procrastination can be useful, because if you are honestly not ready to start a piece or even ready to do the research for a piece, then put it off until you are. Obviously, I'm not advocating that you just put things off all the time. But sometimes you'll have an idea, or the kernal of an idea, and it won't be fully fleshed out, and it's best to keep thinking about the idea, making notes, improving the idea, before going ahead and creating something that just isn't ready to be created.
Writers block can seem uncontrollable and frustrating.
Write a letter to a friend, make up a grocery list, make a list of things to do the next day. Maybe even get out your favorite book and retype a great paragraph. These are all exercises that not only get the fingers moving again, they actually might give you an idea for a column or article. I've read dozens and dozens of books on the craft of writing, selling and marketing writing, bios of writers, non-fiction on the history of publishing, books that have advice from famous and not so famous authors and screenwriters and teachers, books on queries, formatting, how to get a job in publishing, etc, etc, etc. But there comes a time where you can't read anymore about writing.