See if there’s a pinhole near the dvd tray. If so, straighten a paper clip and slowly insert the end of the paper clip into the hole. This will force the tray open.
The ol’ paperclip trick described in the first answer is surefire. It doesn’t necessarily solve the underlying problem but gets it open. I always keep a bent paperclip handy and recognize geeks by the sight of a bent paperclip near their computer.
Thanks, @SABOTEUR, @njnyjobs and @anartist (I started following the Q because I have the same problem with an old 5-disc AV system CD player). I’ll have to look for the hole on the tray if there is one.