Screen goes black during install

reinstalling over a current boot system drive (not wiping the drive first) on my Mac Pro. Running installer from an 8gb usb flash drive. About one hour into the process, after two automatic restarts, the screen has gone black with no activity showing at all. The cursur is still visible and responds to mouse input so the computer is not asleep/frozen/off etc etc. I'm sitting here waiting for something to happen.

I Expect my only option is to force a shut down and try again. What damage will have been done to my current system? Am I now looking at wiping the drive and starting from the beginning, something I was trying to avoid as I have clients breathing down my neck for deadlines and I've already spent two days diagnosing and cloning drives and preparing for a reinstall.

ANy chance I can try again with the 'quick' process and it will work or am I hosed?

thanks