Computer resetting/reformating itself without instructions

The other day i turned my computer on and it seemed like it had re-formated or reset all by itself without me giving it a command.
All my personal files were gone but my programes (itunes, office) were still there.

I want to know why that happened.
 
"I want to know why that happened"...

So do we ;) So you do have some files on the hard drive, and you can boot into Windows normally? Did you have a power failure or did you download or install something recently? What is this computer? Make and model? How old is it?
 
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