Well I'm not a writer, I'm actually a painter, but I do occasionally run into a similar phenomenon. Sometimes I simply don't have an idea for a painting, sometimes I'll have part of an idea, but not enough of one to invest the time in a painting, sometimes everything is there save for one crucial bit.
I find there are a couple things that often help me. One is to just start sketching a lot of stuff (for a writer this might be akin to just writing a ton of stuff without any aim until something comes out that you like). This is better when I haven't got any idea, I just let lines take form and see if I can start to make something interesting, and if I can then I go back to develop it as a composition with thematic content.
The other thing I'll do, if I've got part of an idea, but not all of one, is go for a walk. Going for a walk often yields good results for me, and it's just a more pleasant use of time than sitting around banging my head trying to come up with something.