IMac won’t restart after upgrade to Yosemite

Zap the PRAM and reset the CMS

A small amount of your computer’s memory, called “parameter random-access memory” or PRAM, stores certain settings in a location that OS X can access quickly. The particular settings that are stored depend on your type of Mac and the types of devices connected to it. The settings include your designated startup disk, display resolution, speaker volume, and other information.

1. Shut down the computer.
2. Locate the following keys on the keyboard: Command ⌘, option, P, and R. You will need to hold these keys down simultaneously in step 4. Turn on the computer.
3. Immediately press and hold Command+option+P+R. You must press this key combination before the gray screen appears. Continue holding the keys down until the computer restarts, and you hear the startup sound for the second time.
4. Release the keys.

Resetting PRAM may change some system settings and preferences. Use System Preferences to restore your settings.

How to Reset Your Mac's SMC

The method for resetting your Mac's SMC depends on the type of Mac you have. All SMC reset instructions require shutting down your Mac first. If your Mac fails to shut down, try pressing and holding the power button until the Mac shuts down, which usually takes 10 seconds or so.

Mac portables with user-removable batteries (MacBook and most MacBook Pros):

1. Shut down your Mac.
2. Disconnect your Mac portable from its MagSafe connector.
3. Remove the battery.
4. Press and hold the power button for at least 5 seconds.
5. Release the power button.
6. Re-install the battery.
7. Reconnect the MagSafe connector.
8. Turn your Mac on.

Mac portables with non-user-removable batteries (MacBook Air and some late 2009 MacBook Pro models):

1. Shut down your Mac.
2. Connect the MagSafe power adapter to your Mac and to a power outlet.
3. On the built-in keyboard (this will not work from an external keyboard), simultaneously press and hold the shift+control+option+power button. Release all keys at the same time.
4. Press the power button to start your Mac.
5. Mac desktops (Mac Pro, iMac, Mac mini):
6. Shut down your Mac.
7. Unplug your Mac's power cord.
8. Press and hold the Mac's power button for 15 seconds. 9 .Release the power button.
9. Reconnect your Mac's power cord.
10. Wait five seconds.
11. Start your Mac by pressing the power button.

Alternative SMC reset for Mac Pro: If you have a Mac Pro that isn't responding to the normal SMC reset as described above, you can force a manual SMC reset by using the SMC reset button located on the Mac Pro's motherboard.

1. Shut down your Mac.
2. Unplug the Mac's power cord.
3. Open the Mac Pro's side access panel.
4. Just below the Drive 4 sled and adjacent to the top PCI-e slot is a small button labeled SMC. Press and hold this button for 10 seconds.
5. Close the Mac Pro's side door.
6. Reconnect your Mac's power cord.
7. Wait five seconds.
8. Start your Mac by pressing the power button.

As you have seen, all Apple products actually run even when you power them off. On iOS, the battery keeps time and on Macs, the SMC is responsible for knowing when the power button is pressed and if the Mac should start itself up due to a scheduled wake or power on being saved in the NVRAM.

If you called for service, Apple would likely focus on several things:

1. Have you checked that the firmware and software is up to date.
2. Does this happen every single time, or is it an occasional thing.
3. Have you disconnected everything (including the mouse and keyboard) and tested to see if it's in any way related to a peripheral.

Assuming you can reproduce this at will (it happens every time), you would probably take steps to reset the SMC to ensure it's getting proper readings from the temperature sensors.

• Intel-based Macs: Resetting the System Management Controller (SMC)

If after spending the 10 minutes it would take to read through each and every step (I feel it's important to perform the steps in order - even if they seem unrelated or unnecessary), I would expect them to want to run diagnostics to physically open the Mac to check the sensors and wiring. Your symptoms sure sounds like the SMC either is stuck in some loop and not reading the temp sensors. Failure to control the fans properly could also be a hardware issue where the SMC is receiving a signal where the correct action is to keep the fans not only spinning, but spinning at an elevated level.