I would say that your DVD drive is either dead or some serious dust has accumulated round the laser. Not too sure how long you've had the machine so a few things to try would be:
1) try cleaning the drive with a generic CD cleaner, it should start cleaning it straight away as the head moves underneath to try and read the disk, if it recognises that then let it do the honourable thing and clean the lens. Try another disk afterwards and see if it picks it up.
2) If the above hasn't worked then I would suggest a new DVD drive, again dependent on how old the machine is will affect the availability of parts, ebay I would say is a good bet to get you started.
Best of luck
Urban Kaos, November 2007