If this is the setup for a joke, "reinstall the DVD driver."
If not, it will depend on the make and model of the computer and the operating system. You are running Windows 7, yes?
What are the make and model of computer or, if home-built, what components did you use?
Did the DVD drive work before? If so, what happened since then? Did you re-install the OS?
Do you know how to look in the Device Manager and give us information about the state of the device?