Well, it happened to me yesterday. Driving out for some shopping, I got an idea and out came a couple of verses that were at least a start. I tried singing it a few times, thinking that I would remember it later, kind of like that ear worm we get sometimes.
When I got home, I tried to resurrect things, but it was...... nothing. I did remember the hook line that triggered things, but I'm going to have to work on trying to flesh out something.
Had I been at home, I could have fired up the computer or grabbed the H4n and at least gotten a better start.