That's pretty weird! I wonder if something has changed with the recent patches? I couldn't find anything about it on the bug forum. It used to be a requirement for the Best Selling Author aspiration when the game first launched but the aspirations have all been changed since then.
Since that change sims hardly ever get writers block now. It's too late here now but tomorrow morning I'll fire up my game and see if I can get the same thing to happen. That way we might be able to find a work-around.
Something is obviously not working as it should though as usually I can keep my sims writing pretty much all day and they would never get to the point where they couldn't write.
I know its obvious but travel your sim away from the lot and back and see if that helps. I know you delete your cache files too so I won't suggest that!
And make sure your sim is inspired while writing.