E152FP Color Monitor Problem

Hi there I am new to this board and I am not very computer literate.  I have a Dell Dimension 4600 and a Dell E152FP Color Monitor.  My Desktop was broken and being fixed, so I was using the monitor on my laptop (dell of course).  However, when I received the desktop back, I unplugged the connection from the laptop and plugged into the desktop.  I switched on the desktop and waited, the green light on the monitor switched on for a few seconds, then changed to amber.  I pressed the menu button on the monitor and it told me that the monitor was "In Power save mode" and to press any key on the keyboard.  I did this, but nothing happened.  I removed the cable from the back of the computer showed the test screen, I then plugged the monitor back to my laptop and it worked there.  Did I do something wrong...