I have a fairly new iMac Pro 5k got in Jan. I turned on and screen was black, apple logo would pop on and then load then half way thru then turn black. Restarted several times, no avail. We even did a disk repair, then reinstalled and that didn't fix anything, after reinstall, screen would stay black at startup. I was on phone with support for over an hour, end up having to make an appt to bring it in.