DVD Player not working?

I have a complete entertainment system I guess is what you would call it .There's a tape player a VCR a volume control and a 5 disk DVD player .Now Ill be the first to admit in very technically illiterate but I read the trouble shooting Manuel and it said error code 31 meant the DVD was not in properly its in properly its clean but it keeps showing up either error 3100 or keeps opening its a Sony DVD player and was working fine and then just stopped .I know the DVD works because I played in on my pc .All the wires are hooked up properly in the back .Its about 5 years old any suggestions on how to fix it or what might be wrong.