Personally I tend to find the root of a block I have is that I don't like what I'm writing and don't know where it's going, particularly the latter. Without some sort of map of what happens when I just can't write, so that comes down to planning. I've never had anything which wasn't carefully planned get past the 25k mark.
Take the fanfic I worked on after the last one I posted here, the idea seemed sound (I still sort of like it) but it just wouldn't go anywhere. After
cough words I gave up halfway through the fifth revision and looked for fresh inspiration, in part the scale was too big, I prefer writing at a smaller level, so it just didn't suit, I didn't enjoy it so there was a block.
So it comes down to finding an idea you like, for setting/plot, character and themes. For this I browsed around:
http://uesp.net/wiki/Lore:Main_Pagehttp://www.imperial-library.info/ (particularly the maps and history sections)
and noted down things which seemed to fit what I
wanted - room to play around, good scale, right setting, &c.
For example looking at the map I found Stirk - ever heard of it? I hadn't and there seems to be almost no lore. So what's there? Does someone come across it? Deliberatly? Why were they sailing there? ... That's an idea which never made it. Another was looking at the timeline and the origin of the Mages Guild, and there were others. So I gave them all a couple of days to brew and one was a clear winner, I put together some characters and a timeline then started writing and after pushing the first 10k (which will be a bundle of fun to edit seeing as my editor was off for it) it all started flowing.
So in short decide if it's the content of what you're writing which is putting you off and if so, however much you like it, ditch it and find something else. Then just write and push through until it clears.
EDIT:
http://hollylisle.com/index.php/Writing-Li...ng-fun-yet.html , she often has fairly good points, it's a good site to check out too. Her other writer's block article has something I have also used - write something you normally wouldn't dream of, it shocks the internal editor into submission.