Optiplex 780 works fine with dell monitor when switch to 42 in panasonic plasma

Just got a refurb 780 small form from dell. It works fine on the 23" monitor that came with it. I have a 42" panasonic plasma monitor that I have used on many other computer in the past. When I hook up the 780 it boots up and when it should put up the desk top the monitor show no signal. Whats up with this? Dell had me update the video drivers and reinstall windows 7. I am about to send it back but will give dell another chance..