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.