Your Quadra may be sulking after having had all of its buttons pressed at once. Reset the MLB by removing mains power (but leaving the mains cable connected to a switched-off power point, and removing the PRAM battery for half-an-hour. Then reassemble the Mac and try it after you have ensured that the PRAM battery is at least 3.3V in circuit. You will lose all of your PRAM settings this way, but you will get the MLB back to square one.
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