Wow, I just lost a lot of work. I was a moving some wires around in the board file (lining them up, not even making new connections) and now when I open that file, I get the following in a dialog box:
“ABORT: Unkown objectType (69)”
I can erase the BRD file, open my schematic, and then switch to board, but of course I’ve lost all of my artwork.
I have some intermediate files that I can revert to, but it had been a while since I saved.
I guess I could look at it as more practice 
ooh, I just remembered a zip file of the project that I emailed to myself last night. Not as bad as I thought…
always, always, always have BOTH the schematic and the board open at all times. Otherwise they get out of sync. I consider that the worst aspect of eagle. no guard rails.
the thing is, I’ve always heeded this warning, and they were both open at the time.
I suspect that it had to to with me only having the top, bottom, via, and pad layer turned on. This was the only thing out of the ordinary that I was doing, as opposed to my limited use of it over the past few weeks.