|Possible reasons to check out:|
1. BIOS not detecting hard drive - Go into your BIOS (F2/ Del /Esc key at boot will normally get you into your BIOS) to see if hard drive is being detected or not.
Is your BIOS detecting the hard drive?
2. Your hard disk has been damaged - If your BIOS is NOT detecting the hard drive than this is a likely possibility
3. The MBR has been possibly corrupted - If your BIOS is detecting the hard drive then you can use your CD to boot into Recovery and try to repair / fix the MBR.
☺ When everything else fails, read the instructions.