I have a Mac mini late 2012 model with macOS High Sierra. Several days ago I upgraded my monitor to a U2715H model. I connected it with a DisplayPort cable to my Mac.
U2715H is a fine display. Everything was good. Until I had gone for a few hours and returned to find myself unable to switch the darn thing on. The display said «no signal», then «going to sleep», and here I was, staring at the brand new pitch-black display and wondering where I put its warranty.
I never switch off my Mac and never let it sleep. It works as a server for my local network. So, what’s happening? After (non-configurable) timeout the display goes to sleep. And when it wakes up the message somehow fails to reach the Mac. The only way to make them be friends again that I found was to reboot my Mac. Not the ideal solution, to say the least.
This problem with Dell displays and Macs seems to be somewhat common if you google for it. I’ve tried every advice and trick I found to no avail. But then I somehow thought up to change my old trusty DP cable that served me for quite a few years with my previous Dell display without any problems whatsoever to the one that was bundled with my new display. Lo and behold, it worked. The trouble’s gone for good. I love Dell displays again 🙂
The new cable is DisplayPort 1.2 certified, my display manual says. Even though I don’t use DP 1.2 mode maybe this fact does make the difference.
I have posted this for everyone who is affected with the issue. The answer to your problem may be simpler than you think 😉
Just try another DP cable.
P.S. My English is far from perfect, it’s not my native language. But anyway I hope my post will succeed in helping another person who is staring at a blank pitch-black Dell display…