Unfortunately this is most often a hardware failure (bad motor, worn gear, loose belt, etc in the ejection mechanism, faulty switch or sensor, or just an obstruction by a disc that is out of place)
There are a few things you can try:
First unplug power for at least an hour, then press and hold the power/on button while plugging back in, and keep holding for another 60 seconds. If it still doesn't open, repeat the process but with the eject button instead of power.
If it still does not open, pick the unit up and shake/tilt the player side to side gently, hopefully dislodging the stuck disc inside that could be preventing it from opening, then try to open again.
If none of these help, then there is an internal hardware defect, which will need to be repaired.
If it gets to that point, I can advise you on service options.
Let me know the results, and I'll advise further, if needed.
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