The DVD pickups could be bad. You can try cleaning the lenses with a soft cotton cloth or swab. If that does not fix it, and you tried a few disks that work elsewhere, you need to get a new player.
And as above, are these burned DVDs? If so, not all players can play them, and not all those that can play them can play all the formats of blank DVD disks you can get. RW -R +R, those make a difference.