I recently bought a used Mac Pro Mid 2010 with Eight core 2.4ghz. They have fitted it with a 480gb 2.5" ssd, 4 500gb drives, Nvidia GTX 970 and 40gb ram. High Sierra OSX installed.
It all worked fine for two days till I did a security update today. When doing the update it prompted it wanted to shut down and reboot, but it didn't seem to do that. I waited a while and turned it off by pushing the power button. When i started it up again both screens are all black. I do here the start up chime every time tho. I've tried all different things to try to resolve this.
Re-setting SMC, NVRAM etc.
Tried booting up in safe more, recovery mode etc, nothing on the scree, it's still all black.
Start Mac > Press command + option + P + R simultaneously > Stop pressing after hearing open sound four times, and iMac screen will be normal.
Press shift + control + option + power button simultaneously for one or ten seconds. After stop pressing four keys, push power button to start Mac.
Probably did try other things to that I can't remember.
The original GForce gpu was still in the machine so I tried using only that one to with no luck.
Replace the little flat round CR2032 battery.
Nothing seem to help.