On the front of the drive, look for a teeny, tiny little hole (it's often in the lower left area, as you're looking at it - but varies by manufacturer).

Straighten out a paper-clip, and stick it into the hole. The tray should eject just a little, but enough to grasp it and pull it all the way out.

Then try another known-good disc. If it works fine, that stuck disc might be munged up now. If it doesn't work fine with the known-good disc, your drive may have reached its end of the road. But before you ditch it, try re-installing it, as Beirut suggest above.