Since you don't specify the model, you are unlikely to get a comprehensive answer. Check the user manual for "manual disk eject" or something similar. On some trays, there is a pin-hole you can insert (e.g.) an unfolded paper-clip end into, which manually unlocks the tray, but this is not universal.
Best bet is to consult the manufacturer's documentation.
Sparks (the Plumber)
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