For anyone that may encounter this problem in the future, I'm happy to report that I was able to find the deleted file under Edit> Umbrella> Show Deleted Items.
I wish I'd known to look there earlier (never lost a file before, and the umbrella menu is a little counter-intuitive to associate with deleted files, so I missed it the first few rounds of panicked searching). I also still have no idea what happened to my file in the first place, but crisis averted!