List of possible reasons in addition to the abovementioned also include:
Damaged hard drive
BIOS does not detect the hard drive
Corrupt Master Boot Record (MBR)
You have a CD disc in and your BIOS is set to boot from CD
You have left a thumb drive in a USB slot and its trying to boot off that
If its none of the above, maybe your bootsector has been damaged.
BOOTREC.EXE is a WinRE tool (available in Windows Vista and later releases of the
Microsoft OS) used for troubleshooting this type of error.
You possibly need to write a new boot Sector
use as follows(in WinRE):