hello there. the fastest thing would be to manually open the case and make sure that it is connected (sata or ide) to the motherboard.
if its not then obviously, and carefully, plug it back in.
if it is and still no good, then the next thing id say is make sure you have the latest drivers for it.
look for the manufacturer of the drive so you know what to look for as far as the drivers.
if the computer is a store bought pc, then go to the manufacturers site, go to the downloads section, and enter the information for your pc (assuming its not too old) then something should come up in the downloads page, rummage throughout the page, and somewhere on there, should be a download for hardware, just look for the one you need and download.
thereafter, its a simple click and install.
if none of this does it for you, then it may be time for a new one.