There are two scenarios where this may occur.
1) The hard drive does not have a functioning operating system installed or it has been corrupted. If this is the case, you can try and to a repair on the drive or restore from a previous back up. Otherwise, you will likely need to perform a clean installation of the operating system.
2) The boot order of your computer has something other that the hard drive listed first and that device (DVD drive/USB) does not have an operating system installed. The computer is trying to boot from that instead of your hard drive. If this is the case, simply remove that device or change the boot order in BIOS so that the hard drive is the first device listed.
-Wolf sends