Writer's block.
It exists, it's real, it's out there, lurking in every bush, hiding behind every tree, and it'll stop you up faster than a one-kilo block of gorgonzola cheese on heavy Russian black bread with no water.
Constipation is far worse, by the way.
Okay, seriously, I'd agree with the poster who said that there were stumbling blocks. I've had many in my work, so I always go back to the beginning, read through the manuscript and find out why the character does what he or she does or why he or she feels that way about the situation or another person or something. After doing so, I can go to the bathroom again, metaphorically speaking.
(No, it doesn't mean I crap all over my keyboard. Major ick).
Now, there ARE times when I'm lazy as hell, but that's me and no one else. I would never presume to speak for another writer. Just for me, there are times when I just don't want to write and leave my manuscript for another day. However, once I get back to it--and if I'm really excited by the concept then I'll WANT to get back to it--the ideas come once again and it's off to the races.
Or the bathroom, whichever comes first.