|Make sure there are no USB memory sticks or a CD left in the tray. It may be trying to boot from those.|
If it's not that, try getting to your Bios which will be something like F2 or Delete when the computer is booting. In the Bios look for Boot Sequence or something similar.
It will be the order of boot options, make sure the Hard Drive is set to boot first, then floppy/DVD drive/USB etc..
If still nothing there's a possibility your Hard drive has died or certain files have been deleted/corrupted causing it to fail boot.