Why won’t my iMac 5K display wake from sleep?

If you’ve one or more external displays and applied the Sierra 10.12.4 update, this is a known bug. Disable display sleep, relying on a screensaver to darken your displays instead. This should be fixed in 10.12.5. If it’s your only display, run Diagnostics to see if it’s a graphics card issue. Details are at apple.co/2qVmCDB.