Some optical drives have a manual tray release (in case of power cuts etc.). Have a look on the front of your system for a small hole immediately below the disk tray (or failing that anywhere near the tray door) and if you find one, use a bent paper clip, or similar, to poke into the hole. This should pop the tray open. The thickness of the wire used to make a standard paper-clip should give you an idea of the sort of size of hole that you are looking for.
Failing that, unplug from the power and take the case off the unit; you should then be able to get to the tray opening mechanism.
(Just a thought: I seem to vaguely remember that some tray mechanisms use rubber bands/small belts and it may be that yours has slipped or broken.)