20" Imac has white screen and freezes at start-up

Question

my 20" imac will not start....help...All I get is a white screen and tone at start-up then it stays white. I tried hooking up a bootable firewire drive and pressing the alt/option key at start up nothing. I also tried the command/option/p/r at start up, it will reboot to the same screen. I also tried running the hardware diagnostics at start-up...no response. Computer was purchased on Nov .4 2007, warrenty just expired.
Help......Please....

Answer 1

I had the exact same problem today. My iMac 20" (iMac 8,1) would't boot any more, staying stuck on a white screen immediately after the normal start-up chime. Nothing worked (no safe mode, no DVD boot, no disk selection icon when starting with alt pressed, no Apple hardware test, no parameter RAM reset, power management chip reset had no effect, no verbose mode, not even open firmware).
I browsed the forums, but what I found didn't help, so I was about to stop trying and have it fixed by Apple. But my son told me he had noticed that after "a long time" (he's 4...), a folder with a flashing ? would appear. I figured that wouldn't happen in case of a severe hardware failure, so I tried one last thing: I hooked up an old PowerBook in target disk mode via Firewire and restarted the iMac. I tried several times (with and without alt pressed), but nothing happened (when pressing alt, a functional mouse pointer would appear, though).
Then I restarted a last time and left the iMac sit there on its white screen for about 20 minutes... and that's when the miracle happened: it finally booted off the target-disk mode PowerBook's system!
I ran Disk Utility, which saw the iMac internal HD as disk2s0 (or something like that), and launched the disk repair process. After about three error-ridden verify-repair cycles, the green ok message finally appeared and the volume mounted as if nothing had happened. I rebooted off the internal disk without a glitch, and I still have to find damaged or lost files.
I guess I was lucky that my HD problem was benign enough to be cured by Disk Utility. But even if that had not been the case, at least I'd have known that the HD failed, and maybe I could have saved some files using recovery software. Whatever the case, if you've got an external bootable drive, it's definitely worth trying before handing your iMac over for (usually horribly expensive) repairs.
Good luck!