Is there a reason why screen color turned yellowish while the LCD power is on when rotating? I know CRT when rotated while its power is on becomes discolorized, but I'm surprised it has same effect on LCD. What could be the reason? My google-fu is failing me, cannot find any technical reason for the discoloration.
The most likely explanation is that the blue wire in the video cable disconnected when they pulled the monitor. Make sure that it is securely connected at both ends. If so, then try a different cable because it may have a break inside the cable.
I had a similar problem with an old CRT monitor, but mine turned purple-ish because the green wire was bad and jiggling it would make the correct color come and go. Recently, I turned an LCD to portrait mode and the audio cable got bent because it did not have enough slack. You really need to remember to be careful of the stuff in the back when doing things like turning.