Sorry to post this here but you're all so lovely and helpful...

[Allen_Wicks@adobeforums.com]

My dad has done that with the brightness multiple times...

Not being able to start from CD is a very bad sign.

[Bob_Janusz@adobeforums.com]

I suspect the video card also.

The same thing has happened to my upgraded Power Tower Pro. Twice in less than a year.

The only thing that brought it back was to re install the old Twin Turbo video card. After a successful boot and shut down, put the Radeon Mac Edition back in and all is well again.

???

[graffiti]

Did you ever get it figured out Lance?

[LanceBellers@adobeforums.com]

Sorry for slow update but as I mentioned, he lives a few miles away. Anyway, managed to get over there, armed with a new 3.6V battery. My brother was there too who has hardly any experience of Macs but seems to have a knack for thinking stuff through.

So, I open up the PowerMac 8100 case, search around for where the battery is. Can't find the bugger. Get torch, poke around, nothing. Thinking may have to lever (somehow) the motherboard off. Then my bro spots it, in black casing, on back of main motherboard section. Put in new battery, start it up (attached to monitor)... nothing. Again, as before, Mac sounds healthy but nothing on the screen.

Try this and that, extensions off, usual tricks, nothing.

Open Mac again, try pushing in videocard nice and snug (maybe it was loose), put Mac on its side to get gravity on my side as I slot video card in, feels better, restart... nothing.

Try wiggling cables, try changing power leads into both monitor and Mac, no joy.

Try a few more times, ring another pal (who might have old monitor), no joy.

Eventually, give up, have cup of tea, talk to step-mother. Meanwhile, brother is putting back the external CD-ROm on top of Mac and plugging in the modem to back of Mac again. Amazingly, while I'm not even watching, he restarts it and it comes on!

Hurrah! Couldn't believe it. Today I get email from my dad who has used it to book a hotel for a long weekend on the coast, he's v happy.

Many, many thanks to all those who helped and gave advice here :-)

[Ann_Shelbourne@adobeforums.com]

Possible answer to the mystery:

Did you brother take the sensible precaution of pulling the power cord from the wall while he worked on the machine?

I have found that actually yanking the cord; leaving it unplugged for a while; and then re-plugging and booting somehow resets some internal circuitry.

A perfect example of this: unplug and take Mac to the repair shop.
Repair Shop plugs it in …… "What problem did you say you were having……?"!