This is most likely caused from your eject button being stuck in place.
Unplug you console from the back (the main power cable), push the eject button (softly) several times to make sure it is in place, then plug your console back in and try. (This is for the older style consoles).
Now, if you have a NEW style console (slim), then you will most likely have to call Microsoft (1-800-4MYXBOX) and see if they can assist you further, most likely recommending a repair.
Sorry for the misfortune, have a nice day!
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