Try resetting the PRAM.1. Shut down the computer.2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need them in step 4.3. Turn on the computer.4. Press and hold the Command-Option-P-R keys before the gray screen appears.5. Hold the keys down until the computer restarts.6. Release the keys. ________________
Hold the power button on the back for about 15 seconds. Also if that is a new MacPro that is a Thunderbolt port not a Mini Display port are you sure you have the right adapter?
I aklready told you how to shut down:
Hold the power button on the back for about 15 seconds.
If it has it's own Thunderbolt cable then yes that should work.