I’ve had this problem recently on my iMac Late 2013 27inch. An external monitor on a Thunderbolt 2 to DP cable works fine so I can still see what’s going on and so clearly the GPU is fine. So far it’s happened about 3 times and I’ve got it back to normal (perfect) screen this last time by putting a full screen of white onto the screen. Gradually (over the course of a day) the screen basically repaired itself. Then overnight I left it running with a screensaver called Drift and this morning it is back to perfection. (The previous times I had these vertical lines (blue/black over no picture) it kind of fixed itself). BTW I tried the standard PRAM-type fixes to no obvious fix but maybe they helped in some way. Weirdly after the last power-off and restart, I was seeing an image from a previous application running (ie what I’d been doing before I shut down the comp) burned into the screen which gave me the idea it was a bit like plasma burnin, so I brought up a white screen (I think it was just a mostly white web page) and gradually over time the dim image got brighter and seemed to repair the LCD. In summary, connect a second screen to see what you’re doing and then do PRAM/safe mode power cycles, and then try ‘resetting’ the LCD itself by making it go all white, and then give it a ‘wash’ with the Drift screensaver (black and silver). Hope that helps someone.
+
I’ve had this problem recently on my Late 2013 27” iMac
+
+
An external monitor using a Thunderbolt 2 to DP cable works fine so I can still see what’s going on and so clearly the GPU is fine.
+
+
So far it’s happened about 3 times and I’ve got it back to normal (perfect) screen this last time by putting a full screen of white onto the screen.
+
+
Gradually (over the course of a day) the screen basically repaired itself. Then overnight I left it running with a screensaver called Drift and this morning it is back to perfection. (The previous times I had these vertical lines (blue/black over no picture) it kind of fixed itself).
+
+
BTW I tried the standard PRAM-type fixes to no obvious fix but maybe they helped in some way.
+
+
Weirdly after the last power-off and restart, I was seeing an image from a previous application running (ie what I’d been doing before I shut down the comp) burned into the screen which gave me the idea it was a bit like plasma burnin, so I brought up a white screen (I think it was just a mostly white web page) and gradually over time the dim image got brighter and seemed to repair the LCD. In summary, connect a second screen to see what you’re doing and then do PRAM/safe mode power cycles, and then try ‘resetting’ the LCD itself by making it go all white, and then give it a ‘wash’ with the Drift screensaver (black and silver).
I’ve had this problem recently on my iMac Late 2013 27inch. An external monitor on a Thunderbolt 2 to DP cable works fine so I can still see what’s going on and so clearly the GPU is fine. So far it’s happened about 3 times and I’ve got it back to normal (perfect) screen this last time by putting a full screen of white onto the screen. Gradually (over the course of a day) the screen basically repaired itself. Then overnight I left it running with a screensaver called Drift and this morning it is back to perfection. (The previous times I had these vertical lines (blue/black over no picture) it kind of fixed itself). BTW I tried the standard PRAM-type fixes to no obvious fix but maybe they helped in some way. Weirdly after the last power-off and restart, I was seeing an image from a previous application running (ie what I’d been doing before I shut down the comp) burned into the screen which gave me the idea it was a bit like plasma burnin, so I brought up a white screen (I think it was just a mostly white web page) and gradually over time the dim image got brighter and seemed to repair the LCD. In summary, connect a second screen to see what you’re doing and then do PRAM/safe mode power cycles, and then try ‘resetting’ the LCD itself by making it go all white, and then give it a ‘wash’ with the Drift screensaver (black and silver). Hope that helps someone.