It means the BIOS cannot boot from the drive that contains your operating system. This can happen for any of a number of reasons:
-- Disk failure
-- Corrupted or incorrect BIOS settings
-- No power to the boot drive (bad or disconnected power connection)
-- Bad or disconnected data cable or ribbon to the drive.
At start-up: Enter your BIOS and locate the screen showing the boot devices. Set boot from the drive that holds your operating system. Save the BIOS settings and Exit. The computer will restart, and should boot to the operating system.
If it does not, disconnect power cord from the wall and then the check the data and power connections to be sure they are properly connected to the drive and the motherboard. After you have confirmed they are properly connected, plug-in the computer power cord to the wall and restart.