That depends. Does it show zero bytes total, or zero bytes available? Right-click on the drive with the disc in it, select Properties, and see Used space and Free space.
If it's a DVD that you know has data on it, like one you bought, try it in a drive in another machine to see if your drive has failed.
If it's a blank writable DVD, and you put it into a read-only DVD drive, then it's useless. So please provide just a little more information: is it a commercial DVD, one that you wrote, or blank? Did you try it in other machines? Do you have a DVD writer, or only a reader? Do other DVDs work in that drive?