What operating system are you using? Did you re-install your operating system? Did you open up your computer and changed any components? Have you simply tried restarting your computer? You need to provide more information for us to give a good response.
Here are a few possibilities:
(1) The "driver" for your DVD drive may have been corrupted. Go into your BIOS and check to see if it's detecting your DVD drive, if it is, then it's probably a driver issue. If you have a pre-built system, go to the manufacturer's website, find your model number and download the latest driver. If you had manually installed the DVD drive, then you can go to its manufacturer's website and download it. You can also find the drivers here: http://support.microsoft.com/fixit/
(2) IDE/SATA cable and/or power cable of your DVD drive may have gotten loose. If your BIOS is not detecting the drive, then it could be a connection issue. So, pull out the IDE/SATA and power cable, and plug them back in securily and properly.
(3) DVD drive is broken. If your drive is broken, then BIOS will not be able to recognize it. To see if it's your computer or the DVD drive that is not working, you can also remove the DVD driver, install it on another computer and see if it works on that machine.
(4) If none of the above worked, then this link may help: http://support.microsoft.com/kb/982116