Portable DVD player not detecting disk?

Is there any way to fix a portable DVD player that is not detecting a disk? It used to work, and now it doesn't. I got it second hand from my dad (he wasn't using it). I played about a half a dozen disks or so in it. Then one day, I went to hook it up and now it says "No disk" or "Disk not detected" something like that. I tried all kinds of disks I had. I tried a disk that had played in it previously, and even tried a brand new disk! (so, obviously no scratches on it whatever) Still, same message.