2009 Mac Pro Desktop Black Screen Issue During Update

Hello all !

I want to start out by saying any help is appreciated here! I repair computers on a daily bases but I am not super familiar with the Macintosh side of things.

I have a customers computer that has a couple different problems. The first one is fixed which was some funny power management issue with the old power supply. Swapped it out, so the first problem is fixed.

Now the second problem. Keep in mind I don’t have another Mac computer to work with here being as I typically work with Windows computers. (I actually purchased one and have one on the way). I press the power button, and the Mac OS starts into what looks like an update (loading screen with apple logo.) The customer doesn’t recall starting a Mac OS update on their otherwise obsolete system. But it looks like maybe they did. Although she did mention it kept asking her to update. As soon as the progress bar gets to a certain point, about 3/4 the way done. The monitor goes to sleep or into standby mode. (Black screen). I’ve done this several times and determined it’s the exact same spot where the monitor cuts off each time. What it looks like to me is, a macOS update taking place that isn’t compatible since the monitor is shutting off. I also need to save her data, she has some important stuff on here.

So, my question is, how do I revert back prior to this update? Every time the computer turns on, it goes to this progress bar until the monitor eventually turns off. A step by step guide would be great, so I can get this thing out and move onto something I’m more familiar with. Any help or guidance is appreciated!!