There is a simple cause for this. Its a result of failure of the OPTICAL DRIVE, and needs to be either replaced, or SOMETIMES I'm able to take cover off, clean and lubricate areas of the drive, and save the optical drive. it just depends if the drive is going to be cooperative. SO WHY? when you put a cd/dvd in these drives, an arm comes up that prevents another disk from being inserted. One of the many types of failures with these drives, the arm will malfunction and the arm will come up and you cannot insert a disk in the unit, even though it's empty.
If you take the unit apart, you'll see what I'm talking about and you may be able to lubricate this so that it returns to normal function. if you go to my website you can purchase repair and send it to me, or just send to me and ill give you a free estimate