My iMac G5 from a few years ago (the square white one) dies now, with a message about an invalid memory access. It goes straight to an Open Firmware prompt and doesn't even load the Mac OS at all. Am I correct in guessing that the memory is probably the issue, and that if I replace it, we may be able to boot again as normal? That's my gut feeling anyway, so I ordered some more memory and plan to swap it today. Any thoughts? Ideas?
Sounds like hardware failure - like when the car starts crunching on a gear change, you know it's bad even if you're not a mechanic!
Try putting the system disks in and starting up holding down the 'd' key, if you can get to the hardware test, it can confirm what's gone wrong. If it's the logic board it's probably a write-off unless you can solder! If that doesn't work try reseting the PRAM - again model dependent hold down command, apple and 'P' and 'R' keys.
You can have a power supply problem. Try to use your iMac on a friend's house, preferably not near your home. If the power is bad on your neighborhood it is good to test it on another place. Check your AC outlets, UPS, filters, extensions and power circuits where your computer is connected.
This can be a hard disk problem too. I had a problem like this on a PC a few years ago. The power outlet had a problem and the power was flickering. Because of this a hard disk and then a memory module failed and the computer crashed like hell. I solved the problem, replaced the failed parts and never crashed again.
Check for overheat too. See if your imac's openings are unblocked or the mac is overheating for some reason. Check for extreme low temperatures too.