Power Saving mode

I used my Dell 991 monitor to test out another computer.  When I switched it back to the original system it would display a couple of screens as it was coming up then go into power saving mode.  If i move it back to the test system it works fine but when i move it back to the original system it goes back into the power saving mode.  When I first switched it back to the original system i forgot to turn the system off first and thats when the problems began.  Anyone have a solution?