Unplug the units power, unscrew and remove the top. it has 3 screws on the back and I believe 2 in each side. Look to see if anything such as a CD is jamming or preventing the tray from opening.

If there isn't something like a CD physically blocking the door mechanism then the issue could be anything from a slipping drive band a dead motor or something has gone wrong with the mainboard.

The "expected" life of an audio component is 5-9 years. If its past 3 years old its generally not worth repairing as 1 hour labor and a motor for instance would cost you more than the cost of a new unit.

Hope that helps