Your experience really does emphasise the need for incremental saves i.e. "Save As" filenameA.psd ; filenameB.psd etc..... The regularity depends on how you value your time if you have to drop back.
You may also want to take a look at this thread, from just a few days ago, where we discuss using Drop Box and the CC Cloud storage to easily save versions whilst working
In house we all use dropbox all the time. This is an outside contractor, so we can't have him on our main dropbox.
I know he has a save last night at least as he sent me a WIP jpg and he would have save the PSD at that time.
He could have saved out a jpeg without saving the psd, and then it would be lost. Some people don't reboot for a week, they just let the machine go to sleep and then pick up.
Lesson learned, I should think.
If the psd really was saved and then reverted to a week before, it would be a malfunction of previously unheard of proportions. Not saying it couldn't happen, but we'd hear about it.