DVDs won't play if the disc is damaged or if the DVD player is broken or otherwise unable to read the disc. Cleaning the disc will sometimes fix the problem, and a DVD laser cleaner can sometimes help.
Scratches and other damage can cause a DVD to fail to play, but there are cleaning kits and devices that can sometimes repair small scratches. In some cases, DVD players may not be able to play burned DVDs. This is particularly common in older DVD players that were designed before DVD burners were available. Testing a DVD on other players can help users determine if the problem is with the disc itself or with the DVD player.