Dvd Stuck


I'm totally unacquainted with macs... got my first mac (ibook G4; tiger OS X v10.4) a couple of weeks ago. am very exited about it. the problem I have is, that I've already watched a lot of movies, and suddenly this morning my ibook just wouldn't recognize a dvd when I put it into the drive. the trouble announced itself.. first the movies just freezed or made weird pauses, and now the computer just doesn't seem to detect that there is a dvd in my drive... when I tried to run the dvd player application it says that there is an initialization error, that "a valid dvd drive couldn't be found" (error no -70012)... what is happening???
thank you very much for your help...! (I'm desperate)