Try the below mentioned steps to isolate the problem:
1. Power on/Restart the computer and then start tapping the F2 key when Dell Logo appears, the machine will enter System Setup.
2. On the first page that appears see if there is a Hard Drive listed next to Fixed HDD.
3. If the Hard Drive is not listed press F9 key and Load the BIOS Defaults and then restart the computer.
4. If you again get the Hard Drive not detected message, take a technician's help to reseat the Hard Drive(if you wish to do it yourself you can refer to the service manual: http://support.dell.com/support/edocs/systems/insN5110/en/sm/pdf/om_en.pdf
5. After reseating if the Hard Drive is not detected again, you need to replace the Hard Drive.
6. If the Hard Drive is listed in Step 2, run Dell 32 bit Diagnostics on the machine (for more details on how to run the diagnostics check the User's Manual page 64: http://support.dell.com/support/edocs/systems/insN5110/en/sg/sg_en.pdf
7. If the diagnostics comes up with an error code, do reply to this post with the exact error code you get.
As mentioned earlier, for Hard Not getting detected after loading BIOS defaults/Optimum settings, take a technician's help to reseat (disconnect and then reconnect) the Hard Drive. If you wish to do it yourself you can refer to the service manual: