The most common mechanical problems that can prevent a dvd player from playing or reading the discs are the following:
1. The spindle motor not functioning properly (this is by far the most common, if the spindle motor is not spinning the discs then the optical lens can not read them). For the most part it is an cheap part to replace averaging around 25-35 dollars if you can replace it yourself. If you do not know how and have to take it to a repair shop expect to pay 50-75 dollars in labor depending on the area you live in. The best thing is to do some comparison shopping and call the different repair shops and get estimates. The key is to listen closely when you insert the disk into the player, you should be able to hear it spin up. If you hear nothing then that is most likely the problem.
2. The tray could be out of alignment. This is another easy and cheap fix if you have the proper tools. This could happen if you drop or pull/push to hard on the tray (which you should not really do anyway, but always use the open and close button). This is an example picture of the drive assembly with the tray open.
3. The small gears are plastic and often break, this will cause the players other parts to not move and function properly. They are difficult to reach, but inexpensive to replace. It is always recommended that you always order any part from the manufacturer of the device. A visual inspection is usually required to determine if the small teeth have been broken, which requires for the most part taking out the carriage assembly.
4. The small belt drives either break or come off. Again, this will cause the other mechanical parts not to function. They are difficult to put back on as well, so you may need an extra pair of hands if you have not done this before. On average you will usually have 2-3 of them inside your player.
5. The Optical reader has malfunctioned. This is the most expensive thing to replace, so you might consider purchasing another player depending on how much you want to spend and the original cost of the device, unless you can find a great deal on the part you need. You can check to see if the laser is firing with a visual inspection (DO NOT LOOK DIRECTLY INTO THE LASER) put in a disc with the case off and you should see the glow of the laser hitting the disc. You can also see the spindle/spindle motor.
Picture of the optical reading assembly dismantled:
6. The tray also has a drive motor which can fail and cause the cd tray not to open and it also has a track that it rides on (located in the drive assembly). Check these two components if you are having problems with your tray not opening and closing properly or stops working completely. Sometimes, the tray will open and then immediately close on it's own. This is usually caused by the track being dirty or some type of debris is lodged in the track. Cleaning the inside of the player with a can of compressed air will help remove dust and dirt. Blowing out the inside of the device once a month is a good preventative maintenance measure.
CAUTION: When working with any electrical device, you should always ensure safety first. Make sure that the device is turned off and unplugged from any power source. This does not completely prevent electrical shocks because Capacitors will hold an electrical charge for some time even if the device is turned off and unplugged.
The following are example pictures of what Capacitors look like: