iMac (Late 2013 or later): Doesn't start up
After you press the power button on the back of your iMac (Late 2013 or later), the computer might appear to not start up.
This is normal during the very first startup cycle when your iMac is configuring the RAM power settings to be sure that it's maximizing its energy efficiency. This setup period may last up to thirty seconds. This will vary depending on the number and capacity of the RAM DIMMs installed.
Don't push the power button again until you allow the iMac to complete the process. Your iMac will automatically continue to start up once it has completed this task. The next time you start up your iMac (and any time after that), it will not include the RAM power setting configuration and then it should power up immediately.
Note: Installing additional system RAM or resetting the Non-Volatile RAM (NVRAM) will cause the computer to go through the configuration process again.