This happens to me from time to time--it's probably connected to the hard-core driven work that I described three weeks ago. But it's always terribly scary when it happens. The pinnacle of the fear is "what if they realize I'm a fraud?", but there are a lot of intermediate points, like "How will I make this deadline?"
(This fear is one of the big reasons I don't think I'd ever try to earn a living with my writing. I can too easily imagine the extra level of paralysis at the thought that every hour that I'm stalled represents money slipping through my fingers.)
This afternoon, though, after watching the Apple announcement (go iTunes for Windows! Make lots of money, drive up my Apple options!) I managed to get into gear; I was working well, managed to get to a satisfactory intermediate milestone. It's a blessed relief.
Part of me would like to work over the weekend, to get lots more done, particularly since we have a deadline looming large on the horizon--but of course, we're going to Cook Forest this weekend. Perhaps next weekend.