what happened to my Excel file?

I was editing a (rather large) Excel spreadsheet, when all of a sudden, little pull-down menu tabs appeared next to all the column headers. I had never seen this before. Like a lot of weird things that Excel does, it probably appeared when I accidentally hit some key that I wouldn't be able to find afterwards.

I tried to make the tabs go away. Nothing I tried worked. So then I thought, OK, I'll close the file and reopen it. I closed the file, saving it on close (because I had a lot of unsaved data I'd just entered), but when I reopened it, the spreadsheet was gone. The file was there, but there was nothing in it, not even a blank form. (And the size of the file, as listed in Windows Explorer, confirmed this.)

Needless to say, I can't recover it: most recover functions work only for unsaved data, and the instructions for other recover functions all refer to commands grayed out on this file.

What the heck happened?