iMac just started deleted everything while I was using it.

I was casually playing a game from steam this morning when suddenly the wheel of death popped up onto the screen. I went back to the desktop to find steam uninstalling itself. I tried to force quit the action but it was non-responsive so I panicked and pulled the plug. However when I rebooted all my folders are now gone and apps (Adobe suite, firefox, etc) missing from the dock and applications folder. My mac user and all the relevant files within the "my documents" folder are gone too, nowhere to be found.
My downloads and trash bins have been emptied as well. From all accounts my drive has been partly wiped.

I had not intended to reformat my computer and in no way instructed the mac to do this so I am flabbergasted as to why I have suddenly lost 95% of my HDD data.
Photos on iPhoto and all my music is still on iTunes and accessible.

I have run a diagnostic and it says the computer is fine and dandy.

OSX Yosemite 27inch late 2012