First of all, examine the disk and check what type of blockage is preventing the laser to read the disk. If you find dust on it, then you can use a can of compressed air to blow the dust off the disk.
Take a soft microfiber or cotton cloth and wipe the disk. Do not wipe the disk in circular fashion. In fact, you can start at the center of the disk and wipe directly out of the disk. Repeat the process for each portion. Once the disk is clean, you can check it by playing on your CD or DVD player.
If you have little scratches on the disk, then you can wet the cloth with warm water and repeat the wiping process again. You can then wet the cloth with rubbing alcohol or retail disc cleaning solution, and wipe the disk once again. It will help in removing the scratches on the disk.
You can also use plain toothpaste to clear out the scratches from a CD or DVD. Here, you should make sure that you are using toothpaste, not gel, and the toothpaste does not contain abrasive particles such as baking soda, as this can cause more damage to your disk.
Peanut butter, which may not seem a good idea, is basically a good cleaning agent for disks. Peanut butter, a creamy one rather than a crunchy one, contains oil, which can help in smoothing out the surface of the disk.
You can also use a metal polish to get rid of scratches from your CD or DVD.