This just happened to me!
I couldn't see any files in my sent mail, so I restarted and now it is Tabula Raza - like it came out of teh box. I have my applications but ALL OF MY DOCUMENTS ARE GONE!

I guess I'll try my time machine/ time capsule, but I'd love to get the MAC boys input on this without having to pay $170 just to help them with a seriouse OS bug!

To be continued! (working now from my old Dell Laptop... OMG)

Symptoms:
All Data GOne
All Downloads Gone
Boot up screen is original out of the box with no applicaiton icons.
User Installed Applications are correctly listed but launch as if for the first time. All registration and license info is gone.
Mail won't launch.
Safari forgot all my favorites, links etc. and is defaulting to the apple page.
Finder is not showing anything but applications and airdrop and allmyfiles (empty allmyfiles).
Time Capsule andPrinter are still connected (printer connected to time capsule wifi).

I'm paying the $169 to find out what happened before I pull the trigger on Time Machine and maybe make things worse.
Call, Wait 5 minutes, pay $169, wait 10 minutes, Describe symptoms to tech who never heard of this and passes me up to senior tech - holding for senior tech... 20 minutes ..Still holding

Got tech - tried boot with teh Shift key down but now System preferences won't launch so we are restoring the entire OS and Data from Time Capsule (boot with Option key down) - I'll post again when that's done several hours from now.. OMG....