Well, I can relate. Those days when you want to do something musically great, but can't because nothing is in your head. I am currently writing a quartet, and I my ideas ran dry about 5 minutes into the piece. I know it isn't the ending, but i couldn't think of anythin else to do! So I went outside and sat around and stop trying. Then as I was relaxing on the grass. It came to me! In other words, just relax and the inspiration will come back