DVD's are not immune to problems. Mishandling can cause scratching that can render a DVD unreadable. So can environmental issues, like heat and humidity. Get them off of that disc before its too late!

Maybe you used a machine that transferred your VHS tapes directly to DVDs, and you neglected to 'finalize' the disc. Now you've found out that it can only be played back on that same machine, but it's long gone. We can usually rescue this kind of disc.