There a few common reasons why dvd players will not recognize the disk.
1. If the disc to really dirty or scratched the optical reader will not be able to read it.
2. The optical lens could be dirty and will need to be cleaned. If the laser can not see (so to speak) the disc and read the tracks it will not recognize the disc.
3. If the spindle motor does not spin the disc, the optical lens will not read the disc and not recognize it.
4. If there is a malfunction with the optical reader assembly (the laser malfunctions) it will not read the discs. The whole optical assembly is the entire optical lens and circuit board seen in the above picture along with a few other components not seen.
5. The disc tray could be out of alignment. This usually does not happen unless the device has been treated roughly or dropped.
In most cases, a simple cleaning of the discs and the optical lens will solve the problem. If the disc is not spinning, the spindle motor has malfunctioned and it will need to be replace which with parts and labor will cost you roughly between $70-90 dollars (most is labor) depending on the prices in your area that service technicians charge. The most expensive part to replace is the whole optical assembly, which depending on how much you paid for the device, it might be cheaper to buy a new player (average estimated cost of replacement between $ 200-280 most of which is the part unless you can find a great deal some where).
Check to see if your discs are scratched or dirty by holding them up to the light and tilting them back and forth. Try new disc in your machine to see if the problem still persists, if it does then try cleaning the optical lens. You can purchase cleaning discs to do this for you or you can do it manually. You can also purchase things that will help clean your discs as well. For a more detailed description of cleaning procedures, please review my profile where you will find my tips. Check to make sure the discs are spinning. You can remove the cover if you like or you can listen really close to hear if the discs are spinning up. If the discs start and stop the entire process without spinning up after you have cleaned the discs and lens, the problem could be that the spindle motor is about to go out. If the discs are not seated in the tray properly or the tray itself if out of alignment then it could cause the machine to not read the disc properly.
I hope this helps you solve or determine what the device malfunction is. Please let me know if you need further assistance.
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