This issue is often caused by either an incorrect setting in BIOS Setup or a bad boot device, such as a bad hard drive.
When you power up the computer, press either F2 or del buttons a few times to bring up the BIOS setup. then navigate through the options.
If your BIOS Setup has an option to boot from the LAN disable this option.
If you have changed any BIOS settings, make sure to save the changes and reboot the computer (usually the last page of the settings, the quick command "save and exit" is usually F10).
If this does not resolve the issue, try resetting the BIOS values to the default settings (there is an option called "reset to default/factory/optimal settings"), save and exit.
IF this does not work, your hard drive seems to be failing.