First, open the drive manually. (Unfold a paper clip and push it into the little hole under the drawer. That's what it's there for!). If there's a disk in the drive, remove it. (I've sometimes found that a 'rogue disk' has 'locked up' the drive). Close the drawer, reboot the computer and see the DVD drive now works.
If not, delve inside (after switching off the computer and disconnecting it from the mains, of course!) and check that the leads connecting up the drive haven't come loose.
If, having reassembled everything, it still doesn't work, take a look at your BIOS settings to check that the drive is enabled. (Follow the on-screen prompt, at boot-up, to access the BIOS. That usually means pressing F2, F8 or Delete at boot-up).
If you're still getting no joy, it's probably time to replace the drive. If your PC is still under guarantee, take it back to the vendor. If not, the vendor is still responsible for fixing any problem, which is due to an 'inherent fault' (such as a poor quality component), for up to 6 years from the date of purchase. If you can't get the vendor to fix the problem, installing a new drive is simplicity itself. Simply unplug the old one (taking note as to which way round the cable goes - there's a pink strip down one edge) and plug in the new one.
Chris