I spent three whole days in complete panic once, because I'd lost my plot-notes for Grassblades. They were just gone. The entire outline, all the loose ideas for new scenes and plot-twists, every detailed character-backstory - heck, even the page-breakdown for the upcoming chapter - all gone.
I managed to find them again after tearing my room apart looking for them, frantically re-contstructing the bits I could rememember just in case I couldn't find them, but yes. NOT FUN.
And the lesson we learn from this, kids, is to never keep every important piece of writing about your comic in one single notebook with its over torn off, and if you do, don't put it down in a room full of chaotic scattered papers.
I haven't actually lost any work on any of the pages, though it's been close a couple of times - sudden power outages shutting down my computer, etc. When it's happened, though, I've been able to restore the pages automatically - but I hate that sickly feeling of holding my breath and waiting.