I had thought Scrivener was supposed to autosave when the function was turned on. Seems like a realistic expectation, what with the function having been turned on. However when it crashed a few minutes ago (I submitted the bug via the in-app process) I relaunched and found the last two hours of work had vanished… While it has crashed a number of times before (and each time I submit a bug report but don’t hear anything back) I have never lost this amount of work. I’m not a very happy camper at this moment…
What is your setting for auto-saves (under the Scrivener > Preferences > General menu)? As far as I know, there’s no choice over whether auto-saving is on or off - it’s always on - but the issue is the frequency. (From memory) by default this will be set at 2 or 3 seconds, but some users set a much longer period.
Autosave is set to every 2 seconds. Like I said, when I relaunched the application the text was gone. However when I relaunched by double-clicking the .scrv file the text was there. Not sure I understand why that would be the case, but regardless I didn’t end up losing anything (other than my sanity for a few minutes 
Very pleased to hear that you’ve lost nothing (other possibly than some needlessly used-up adrenaline). I can’t explain the Case of the Disappearing Text, but I’m sure that there’ll be someone along shortly from L&L who will probably have an explanation.
My best guess is that you have two versions of the project on your disk, and it is the older one that auto-loaded. It could be the auto-load preference never updated because the last session terminated in a crash.