Cinema Displays 27", blank screen and doesn't come on.

I have two Cinema Displays 27" on a Mac Pro, one display powers up and works fine, the other stays blank with the light blinking three times. Short, Long, Short, meaning my power adaptor is wrong, but switching adaptors doesn't work.

I have read the Apple article about the monitor things it's using the wrong adaptor. But mine I've been using is the correct one for the size and it even works when I switch adaptors and leads with my working display.

Not sure how to fix this. I've tried unplugging everything, and plugging one thing in at a time and restarting to try and isolate any problems.

Any suggestions?