The opening of this kbase article reads like a sad poem:
The Power Mac G4 computer screen is black. The boot tone is not present, the drive does not operate, the fan is running, and LED is lit.
This article is a rare one, in that it wasn’t originally mean for the public, as Apple states:
This article covers repair procedures for authorized service providers on selected Apple products, but we have published them in the Knowledge Base for the general public for educational purposes.
At this time, there are a limited number of repairs that a customer can perform and these are covered under the customer installable parts program http://www.info.apple.com/installparts. If you have not been authorized to perform these procedures, please contact Apple (800-APL-CARE in the USA) or your local authorized service provider to schedule a repair.
This part follows, and is in red (!) text on Apple’s website:
DANGER! This procedure exposes you to high voltages that can result in death or serious injury. DO NOT ATTEMPT THIS PROCEDURE IF YOU HAVE NOT BEEN CERTIFIED AS AN APPLE-AUTHORIZED TECHNICIAN.
Welp.