that's BS man did the previous owner tell you about any known issues with the DVD player?
Did you buy private or car yard?
Private: ring the back and tell them it's not working. Tell them you wont it fixed or replacement.
Car yard: Same as above but just ask for a straight out replacement. You have 3 months warranty.
Some dvd players will have a very small pin hole somewhere were you can push it and eject the disc. Or maybe pull the fuse out and rest the unit.