If your iMac doesn't seem to turn on after you've pressed the power button, follow these steps. Force your iMac to turn off and back on. Your Mac may be turned on but appear to be turned off, even though it's connected to AC power and a working display. Revive the firmware, if applicable. If you're using a Mac with Apple silicon or a Mac with the Apple T2 Security Chip, in rare circumstances the firmware stored in your computer's memory might need to be revived or restored.