why is my imac 2011 cd/dvd player not working any more

Maybe it's dirty... Maybe it's out of alignment... We are not psychic. Why don't you make an appointment at the genius bar of your local Apple Store and have it checked... You might want to try cleaning it first.

The discs may be scratched or otherwise unreadable, or the optical drive may have problems. A surprising number of people have reported success using an optical drive "laser lens cleaner" CD. They are cheap enough to try ( < $10 or so). However, a typical optical drive's failure mode involves its laser diode. They degrade over time and eventually fail, requiring replacement of the drive. They are not terribly expensive or difficult to replace, if desired.