Depends on what computer system is in it, and what computer you are having trouble with.
If its a PAT style computer and the Lmi is the problem, you should be able to go to the 1' x 2' box back on the counterweight and there's a key on the bottom of it that you can turn and it overrides all A2b functions. If that key doesn't work (they tend to get moisture corroded), you can put 12v there, and if that doesn't work, run 12v jumper wires to the shut off coils on the pilot lines. Pilot line coils are generally under the boom between the uprights holding the heel pin.
All this is assuming its not a engine computer that you're talking about in a newer 528. If you are having engine troubles and need to get the boom down (engine won't run), I would grab the tip of the crane with another crane, pull the holding valve on the boom cylinder, and slowly let it down. Holding valve will be in the square block on the boom cylinder, looks like a little round cartridge screwed in there. Oil is going to go everywhere, so be prepared.
The box with the key switch is right on the front of the counterweight, sticking out the bottom of the box. If it won't turn and/or the key is gone, you can just open up the box and jumper it in there.