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If the organ's manual(s) still produces some sound, then that's good news - audio amplifiers have presumably not died.
One key gone dead can sometimes be cured by blowing into the cracks to the left and right of the key (and, with the adjacent keys held down) with an air compressor of the sort usually used to inflate tires.
As for the lower manual not producing any sound... if all the keys on that manual are that way, there could be several causes, but one thing's sure: the cause or the problem or the faulty component is common to *all* the notes. That means either a common run from that manual to the rest of the machine is loose, or perhaps a component that is in the manual that has gone silent, has taken it down.
If the above is not helpful for the problems, then please tell me your zip code or town/city and state, if you would like to call upon repairers of this type of organ that are in your region.