It does happen. Also, sometimes part of it is the idea that now you have to actually WRITE it and that can be intimidating because as long as you don't start, it's this perfect masterpiece in your head. I had trouble last night as well, but forced myself to do some sprints to break myself in. You might need to just take a break for a day and tell yourself that it's fine and you won't even think about doing it until tomorrow. But I wouldn't break for more than a day or two, otherwise, you'll get too far behind.

Good luck! Don't get too discouraged. This is another part of NaNo. Prepping is hard work!