I have a 6-core Mac Pro (the black one) with dual FirePro D500 GPUs and 3 GB VRAM. I wanted to upgrade to El Capitan this morning so I closed all of my apps and shut the computer down (I was going to option boot and use a USB stick to do the upgrade). The computer made the chime sound and the monitor could tell that something was happening (source light changed from yellow to blue), but the screen stayed black. I have tried rebooting multiple times without luck. I tried regular booting, option booting, booting into safe mode, and booting into target disk mode, but it didn't help. Just a black screen; I don't ever see a cursor or a gray screen. I used the thunderbolt cable to plug my MacBook Air into the monitor and it works fine. Any thoughts on what I can do?