What you need to do here is determine whether the problem is the disc itself or the PS3 drive. You can easily test to see if the problem is the disc by putting in another PS3 Game, Blu-Ray, or DVD. If it reads it, then the problem is the disc itself, not the PS3.
It could just be that the disc is a little dirty, or that a fingerprint is blocking the laser from reading the disc inside of the PS3. You can try cleaning it with a lightly-damped piece of toilet paper. You will also want to dry it off with a dry piece of toilet paper. If the PS3 still won`t read it, then the problem may be that it`s scratched. If this is the case, your best bet may be to go to Best Buy to purchase a kit that removes these kinds of scratches. Places like EB Games and Gamestop also have professional cleaners to clean these discs for a mere $5.00.
Now, if there are no discs that work inside your PS3, that indicates a problem with the laser inside the drive. There are means of cleaning the laser inside of the drive, but doing so will break the warranty seal. This means that your PS3 will no longer be covered under Sony warranty if it breaks down in the future.
If this is something you wish to pursue, then try following this YouTube video. If you have never opened up your PS3 before, you may benefit more from visuals than pre-written instructions. :)