By mistake, I hit the template reset button on a custom template. It reset to its default, which, because it doesn't ship with MZ, was nothing ... just an empty textarea. Because it was a new template and I was still in R & D mode, I didn't have a backup.

I had nothing to lose, so I tried my browser back button. Voila, there is was, every bit of it! I clicked save and canceled my crisis.

I'm using Chrome on Win7. YMMV.

lucky you, it was probably in your browser's cache.