If I restore the PC system, will all lost data be recovered?

My PC went blank, though it still works, the internet works perfectly. Problem is that many datas and infos were lost but can they be recovered? This is what might have happened: ''First i turned off the computer while it was on and while I was holding the ''Off Button''. All music files are lost, project files, etc......only some were left :((((((((((. Another problem is that that my PC reads that the hard drive disk is full of files and memories while in the carpets is completely empty. Please i need solutions, advices, suggestions etcc.........