I really really hate when you try to write something, either music or some story, and you just can't, whatever the reason! Your muses just got a day, or maybe a month, off. And then, when you go to bed and you are about to fall asleep, you "visualize" the most amazing music piece or plot you have ever created! And you say: Ok, I'll write it down-because you don't feel like getting up and switch on the pc and record/write anything-and tomorrow... Tomorrow nothing, because you can't recall anything! Even with those doodles you have written down the night before.
*sigh* Yes, it happens to me very often. At least you have a good time enjoying the music or the scene. Or both.