Can anyone explain how using Chkdsk to repair errors can completely wipe a drive clean?

I have used Chkdsk loads of times. I went through the normal way of doing it.... C Drive > Properties > Tools > Error Checking, then said yes to run on next boot up. Rebooted and Chkdsk ran through the 5 stages ok and then went to DOS screen saying.... start in normal Windows or Safemode..... whichever one I clicked, it just looped back to the DOS window.

I took the PC into repairers and they rang me today and said the reason why Windows wouldn't load was because C Drive had been wiped clean!!!

How the fuck has that happened?