This has never happened to me in the 7 years I've been using FCP so I almost can't believe it. I just had a crash after applying a plugin filter to a clip on my timeline. And when I go to restore my project, the last version is from about 4 hours ago. My autosave interval is set to every 30 mins. Is it possible that it has just been quietly failing to autosave for the last 4 hours? I thought if it had any problems saving it would warn me?
I sincerely hope someone has an answer for this so I don't have to re-do the last 4 hours.
I have had this happen to me on the odd occasion where autosave simply has not worked. It is pretty rare but enough to get me into the habit of hitting ⌘ S fairly regularly. I tend to set my autosave interval to be 10 mins, not that that helps when it fails.
Sadly I know this is not an answer to your problem. Obviously make sure your autosave location is where you think it is but other than that I fear that there is nothing you can do if t really has not been saving.