firstly, if your computer has a floppy disk drive, check you haven't got a floppy in there - also check you haven't got a cd in the cd drive or a usb pen in a usb drive - sometimes BIOSes are set to boot from somewhere other than the hard drive first - more often than not the floppy drive if you have one - if you haven't got any of those in then you need to investigate further - it's possible your hard drive has failed - but there are other possible solutions