-new thunderbolt display won't turn on

I have a Mac Pro desktop (mid 2012 2 x 2.4 GHz 6-Core Intel Xeon, 4 GB 1333 MHz DDR3 ECC ) running Yosemite(10.10.3). Took my brand new 27" thunderbolt display out of the box, plugged it in and started my computer. New display stays black. (this is a second display, so I have an Apple LED Cinema display that works fine. Plugged the working monitor into both monitor ports - works fine in either one. Replaced the surge protector. Tried restarting the computer numerous times. New Thunderbolt is still black - beautiful but non functional :-P Now I'm stumped and searching for something else to try - any ideas?