What happened to all my data?

Have you perhaps booted up whilst holding down any keys? You may have inadvertantly entered some kind of recovery/deault mode. Try restarting as a first attempt.

It's very difficult to accidentally format your drive, it requires quite a few steps.

If your data doesn't return after a restart, head to Macintosh HD > Users > (your username) > Can you find your data here? If you can, then you are logged in with an account other than your own. Go to the Apple Menu > Logout and try to access your account that way.

I love the IT Crowd! I'm as relieved as you are that your data is all back.

Do you have your data backed up on an external HDD? There is a risk that the data on your MBA may be corrupted or deleted and you may not be as lucky next time.

Ciao.