|You have not said how many hard drives you have, the type of HDD it/they is/are (ie SATA/IDE) and if IDE how it/they is/are connected.|
First check all the cables are correctly connected on your hard drive/s (power cable/s and IDE/SATA cable).
On boot up enter your BIOS (Del key or F2 key normally) and check if your hard drive/s is/are being detected.
If you have more than one hard drive ensure that Master/Slave/CS jumpers are correctly set. If you are using CS and the HDD is/are IDE, then ensure that the IDE cabling reflects that.
If the hard drive is still not being detected then there may be something wrong with the hard drive/s.
☺ When everything else fails, read the instructions.