I'm having a problem with a 5k iMac 27" (mid-2015). When I turn it on, the backlight comes on, but the screen is completely black and there’s no activity.
An external monitor (plugged in with a Mini Displayport / Thunderbolt to HDMI cable) works, but only after I log in. I can do that by selecting the user and typing in my password blind, although it doesn’t work every time. So I restart it, wait a minute, try to login, wait a minute, then if it doesn’t come on I restart and try again.
When it does work, the computer seems to be working fine. It connects to my wifi (again only after login) and everything is fine, except that the built-in screen is black.
The system recognizes the built-in display and thinks it's working (unlike the Macbook Pro in this otherwise-similar question). It is also powering the backlight.
A few months ago I opened it up and swapped out the HDD for an SSD (with a kit from OWC). It worked afterward, but some of the backlights stopped working, giving a look like stripes of dimness at the bottom of the screen. I needed to order more adhesive strips before I could open it up and re-seat the ribbon cable, so I left it and never got around to it.
It worked for several months. Then one day the screen went black. I turned it off and back on, and the backlight came up but that was all. I tried resetting the NVRAM, reseating the RAM chips, and then gave up and put it in the garage until I had time.
Yesterday I got it out and plugged it in. To my surprise, the screen came on and it appeared to be working fine! I walked away to grab a keyboard, and when I got back the screen was black again and it hasn’t come on since. I plugged in an external monitor and after a lot of messing around figured out I that I can login blind, and once I do the monitor comes on.
I’m assuming there is a hardware issue, probably connected to something I messed up when I swapped the drive. I’m ready to open it up again but I’m not sure what to look for. What are the likely causes of this issue?