Help. Please.

I have a terrifying thing happening and I don't know how to deal with it -- files on my Linux Mint install are having their contents replaced with NULs. Just... everything. Now and then it's an image here and an image there, but things are slowly being eaten. And it seems to happen to completely random things -- not just stuff I have open at any given moment, but random files buried in folders from ages ago.

I've already lost at least a dozen images from my archives and have no idea how to prevent it.

Edit: think I found it after checking for a pattern. Don't hold the power button down unless you absolutely have to, kids.