This can be a number of problems from the disc being dirty up to the DVD drive not working. But fortunate, from what I have read and discover, the main reason being is that the laser is not lining up to read the disc when inserted. But their are many video on YouTube with suggestion on fixing this problem.
My suggestion is if the product is still under warranty, send it back to have the company fix the device; however, if the device is out of warranty, you can purchase another one, replace the defective drive, or try to have the drive repaired. If you still need assistance with any of the suggestion, please send me a help ticket so I can assist you further.