Chosen Solution
Suddendly, when retourning from sleep, my imac shows a grey/black background with some vertical lines in the center. iMac running ok (I can see it with a second monitor) but showing nothing through its display… Any idea of what is happening? What’s broken down? And, after that, how much could cost to repair it? (NVRAM reset already done)
Since your external monitor is working, go for the LED driver board. Look for burned areas or components or cold solders. iMac Intel 21.5" EMC 2428 LED Driver Board Replacement
