Late 2015 iMac won't wake up after sleep

Install El Capitan from Scratch

Backup if possible before continuing.

1. Restart the computer. Immediately after the chime hold down the Command and Rkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Reinstall OS X and click on the Continue button.

Test the new system to be sure it wakes from sleep. If it still doesn't wake from sleep, then make an appointment at the Apple Genius Bar for service. If needed find an Apple Store - Find a Store - Apple.

Kappy-

I'm in a similar boat; however with mine, it is intermittent and I discovered that when hooking up an external HD screen (not Apple) using the thunderbolt and a Mini DisplayPort to HDMI connector, my iMac would also not wake from sleep. However, if I caught it before it moved into hibernation mode? it would still wake.

I have started unplugging that external monitor before sleep and it has been issue free until this evening.

I had to force shutdown and restart after forgetting to unplug. Everything went smoothly- then I unplugged my external monitor before sleep and when returning an hour later, the computer seemingly had shut itself down. I pressed power the chime happened and my mouse pointer appeared on a black screen. The login screen never appeared. I had to force shutdown again. Is it possible that my iMac behavior is similar to Adam's?

So, back to the response to Adam-

I wonder if completely erasing and reinstalling is the ONLY solution to this? I can't imagine it is. Nightmare.

Thanks.

Adam,

I was having the exact same problem and saw Kappy's solution below. However, I wondered if you had any external monitors or devices connected through the thunderbolt port? My issue seemed to appear only when I had an HD monitor attached using a Mini DisplayPort to HDMI cable connector. In my case, the monitor and computer would go to sleep and I wouldn't be able to wake them and would have to resort to the forced shutdown & restart.

Thanks!