Mac Air erased OSX and HD

I have a mid-2013 Mac Air running OS X 10.9.5. Yesterday morning on boot-up there was no familiar Mavericks screen with the dock and all my folders. Instead, I was welcomed by what might be the factory setting screen (with voice prompt) asking for my language preference - it seemed that the OS had been completely erased along with my HD and data with it.

I'd taken a time machine back-up a few days earlier so tried to restore the HD from that, but after 20 mins or so the Mac self re-booted and I was back to square one. After searching a few sites including this one (using my i-Pad), I came across a tip to reinstall the OS X, which I managed to download quite easily (although it took several hours) - once done it was straightforward to restore everything using time machine.

There were no tell-tale signs about what was to come apart from maybe pdf files not being displayed on the desktop (i.e. not as mini-documents that show text and pics but just as a pdf - the red and grey image). But I don't know if this is relevant.

Now, everything seems OK but I just can't understand what might have happened (and I really don't want it to happen again!), so if anyone has any insights that would be really appreciated.

Did you make any modifications to your account before shutting down, like changing your user name? OS X does not mysteriously disappear without some kind of user input. Does anyone else have access to your MacBook Air that could have caused the problem?

Anyway, I commend you for making backups. Keep making them and you might also consider using cloning software as a double precaution. We recommend Carbon Copy Cloner or SuperDuper.

Thanks for the response chscag. I didn't make any modifications that day and there was no other user input. I said to someone that "It was like someone had come into my home and just deleted everything from the Mac"; but this couldn't have happened. It really is a total mystery . . . I'll look at the cloning software - thank you for the tip. Regards HA