iMac mysteriously turned off, won't power back up

The only two things you can really try in this situation is reset the SMC: http://docs.info.apple.com/article.html?artnum=303446 however I would leave it unplugged for 15 minutes. Take out the third party RAM and put back in the Apple RAM as you are going to have to do this anyway when you take it in for repairs. If neither of those work it's time to call Apple support.

George Peters wrote:
The only two things you can really try in this situation is reset the SMC: http://docs.info.apple.com/article.html?artnum=303446

I have a client with the same problem, and this article only refers to 2006 models. It doesn't really say if it applies to Aluminum iMacs.

last week my iMac 24 new model was completely dead. conclusion the powersupply had to be replaced. That's why I surfed this discussion site to see whether it happens more often.
As a technical guy I want to know why??? my local powersupply never caused a problem in the last 4 years

My son has this same issue with his 17inch iMac. This is really devastating for him because he is a college student and does not have the money to get the computer fixed. (the Mac store said it would cost $800)

It seems to me that this type of thing shouldn't happen to so many people. I wish apple could at least make the repairs for this issue a tad more affordable. 😟

Same thing happened to me a few minutes ago (very frustrating); three week old 24" iMac -- I was organizing pictures in iPhoto and all of a sudden it tried to restart, then shut off. Can't get it to restart; unplugged it for about 15 minutes then plugged back in -- pushed start button and got a swoosh swoosh sound and then nothing; tried SMC reset -- no luck -- same sound as before. I have an appointment tomorrow morning at Apple Store. I hope it's just the power supply (and nothing worse). I'll post a response once I return from appointment.

Just back from Apple Store; they believe it is the power supply, but have none in stock so they don't know (still bothersome that a power supply would go out after three weeks). As it would take a week to get a new power supply, they decided to order a new machine. Hopefully the hard drive is not corrupted . . .

I had the same thing happen to my 24" Al iMac.

For me, I just repositioned the machine and it turned off. When I couldn't turn it on I started to panic. I need the machine for my profession, and of course I didn't do a backup last night! <rolleyes>

I did the SMC reset (unplug everything, wait 15 minutes, pray like crazy) and the iMac started back up.
I'd like to know why this happens, what I can do to prevent it from happening again, and what the chances of it happening again are?

I'm off to back everything up and thank my lucky stars that I can do some work today.
Chris

Whilst my machine has not turned itself off the problem with not being able to turn the thing on has. About 2 weeks ago my Imac would not turn on again after powering down. The only way I could get the machine started was to remove the power plug for a moment or two before pressing the power switch - the machine would then start up and boot normally.

Yesterday I removed all of the peripherals with the exception of the mouse and keyboard and booted up (again after having unplugged the machine for a moment). After a normal boot I powered the machine down again. I switched the machine on (still with the peripherals unattached) and the machine successfully switched on and booted up normally.

I powered the machine down and reattached the peripherals. This time the machine switched on normally (without unplugging the power cord) and went through the normal boot process. Has worked without a problem for the last 24 hours - touch wood. No idea what caused this issue to arise - after 2 years of normal operation.

This is somethings that recently happened to my sister's alum iMac 20". We were playing a game over lan, then her computer just shut off. I tried everything, and it doesn't do anything what-so-ever. Its only about 2 months old, I'm using a MacBook Pro 17", and I have no problems with it. Maybe there is a problem with the alum iMac's power supply from the factory?

I'm having the same issue with my new iMac, purchased this past December '07, and which worked beautifully until this week. I haven't customized with any additional RAM or anything. It's been used for the past couple of month fresh out of the box.

However I started noticing a weird behavior a few days ago... It would go into sleep mode as per my power settings but then I wouldn't be able to wake it up unless I unplugged the power cord, waited a few minutes, re-plugged it in and then pressed the power button.

Then last night I decided to try and shut down the computer, which I did via the Apple menu. Now I can't get it to start back up again.

I have unplugged everything, followed the steps found around here to reset the SMU, PRAM and the SMC but all I get when I turn it on is a black screen with the fan going full speed. After about 3 minutes I hear part of the Apple booting sound, there's a flash on the screen and then the computer shuts down by itself. Within 10 seconds it starts again by itself and does the same thing all over again.

During the whole time this takes place, the keyboard and mouse are unresponsive. Since I have both the corded and wireless keyboard/mouse, I tried with both, but nothing. I also tried a different plug outlet but same problem.

Any solution/suggestion?

Hi StudioLaurent!

There is no SMU reset for your machine, it takes the SMC reset only. Following the procedure for an SMU reset will cause the fans in an Intel-based iMac to race. The only way to halt the fans from racing is by then resetting the SMC.

If your problems continue after performing the SMC reset, you'll have to call Apple about it, as there is very little else you can do by yourself. The SMC unit may need to be replaced, or something else.

Same with mine. It's been doing this for weeks. I just haven't had time to bring it to Apple. Sometimes it would just shut down as it's trying to start up. Sometimes while I try to download software updates, screen goes black. It tries to restart, sometimes it starts up on it's own,sometimes I'll be doing something away from the iMac, and it starts up on it's own. It's like a ghost is pulling the plug. I've tried SMC, it works when I can't get it started, but it still shuts off on its own. Thank God I still have my XP PC. Never had problems with my PC. This is my 3rd iMac. I have the original iMac, then the next generation iMac, never had problems with those iMacs. So glad I bought the Apple care plan. Oh, I also ran Tech Tool and Disk Warrior.

The same problem seems to have happened to my 24" 2.8 iMAC also. It was on working fine, then I noticed the screen was blank. Thinking it was asleep, I tried to wake it up. Then I realised the whole machine was off - checked that power cable was plugged in correctly - tried a different power board, and even wall power no luck. When I push the button nothing happens, no sound whatsoever. So I've taken it to the Apple shop. Fingers crossed I don't lose anything.

I just got my mac tonight from an apple store nearby. when i was initially setting it up, suddenly, it turned off and i thought it was just part of the initial set up, but no,.. i did not turn on again. i was so disapointed, coz after a long wait of decision wether to buy a new mac or settle with my old PC, now that I just got it, then it won't work. need to go back tomorrow to apple store and I will insist that they change it immediately or perhaps i will just get my money back.

Message was edited by: musiktenor

For what it's worth, it seems that mine was one time issue. I'm a Mac user since 1998 and this is the first time anything like this has happened.

Still, you may want to have the power supply checked out.