Hi, unfortunately, sounds like something happened to Windows and maybe your hard drive too.
Consider contacting your local ASUS Support to troubleshoot and arrange service if necessary especially if you are still under warranty.
http://support.asus.com/Hotlines.aspx?SLanguage=enIf you don't mind erasing everything and can reinstall Windows, you may want to try this, but may not help.
Otherwise, if you have a spare hard drive you can use to reinstall Windows without getting into trouble with the warranty, try this to see if your hard drive may just be bad.
Good luck!