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Try holding down the CD Eject button firmly, continuously, for over 10 seconds.
If there is a CD in the CD player and you wish to remove it, look for a small 'pinhole' near the drive. Straighten a paper-clip and insert the end into the hole, to depress a small mechanical trip-switch that will unlock the CD door and open it enough for you to pull it open fully and get the CD out.
Try turning Off the BeoSystem and unplugging it, for overnight. Then plug it in, and try it again.
How old is the BeoSystem in question?
The controls would not stop working owing to a fuse, I don't think.
Are any lights or indicators showing any signs of life?
In storage for 15 years - that's as formidable, to electronics, as saying 'it fell down the grand staircase for about 35 feet, bouncing as it went'. 15 years can break down electronics in many way, except maybe if kept in a constant environment and not gotten hot or cold. And even then, sheer time can break down components such as capacitors, causing them to leak or degrade in other ways.
I think that if you do not have good electronics repair skills, it may simply - and regrettably - have to be passed over as 'broken'.