I've seen quite a lot on this topic on the forum, so apologies if this duplicates what has already been explored. I have a beige G3 minitower, with a 300MHz board,Rev 3 ROM, 6Gb HD, and I think 353 Mb RAM. I had it running perfectly on 9.1, and regretably, attempted to install OSX, after which it is comatose.
I think the problem is something to do with the firmware. Currently on hitting start, the screen is totally blank, although at some point it hisses as though it is about to come alive. The gong sounds, and the CD drive whirrs, but I don't even get a grey screen. If I leave it running it will stay dead, although occasionally the CD whirrs. I can't get any other CD to boot now. I have checked the HD, and it seems undamaged with all files intact.
I had the same problem before, and sorted it by swapping the 233 mobo for a 300, and put 9.1c back on. Fool I am, I thought the firmware 'update' and later mobo would work!
What did I do? Well, I read the advice, and, I thought, did a firmware update. Downloaded the B/W G3 update from apple, clicked on it, and it started to open, then vanished from the desktop. I assumed it had done it's stuff, and restarted. 9.1 still fine.
I then put in the Jag CD. Restarting with 'C', I was offered a text message with two options. The first booted 9.1. The second was something like TYY IO (?). Tried this, and all went dead.