Hello, I have a 2006 Mac Pro I use for a video ministry. I tried to install Sierra, which I now know my Mac does not support. I clicked on Sierra install icon, system restarted with no video signal on monitor. I know all works because put in a hard drive with Windows XP (all I had to test) to see what would happen. It worked. I wiped the Mac Pro hard drive using security option zero. Took 7 hours. I hooked hard drive to my Mac Book Pro via usb and drive is 1 TB which is why it took so long. Plus it was a thorough wipe. With drive still plugged into MacBook, I installed Snow Leopard onto Mac Pro drive. Worked fine in Mac Book. I ejected from MB and installed in Mac Pro. Still no signal. Mac Pro has 9 GB RAM, I TB Dell hard drive for OS and apps. And one 80 GB storage drive. It did have Mountain Lion, I believe 10.7.1. And GE Force 9500 GT video card. And GPU present led diagnostic light is not on when pressing button to check. Any help at all will be greatly appreciated. Like I stated earlier, I use this Mac to produce positive, uplifting videos. I am new to MAC word so please be patient. Thank you and God bless.