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I solved this problem simply by pressing the middle grey button on the monitor. This toggles between analog and digital inputs. Since I have the blue cord attached, someone (likely my 4 year old) pressed the button to switch to digital (the white cord) hence the message.
Good job. The technicians should have simply directed you to the manual which discusses the Input Select button.
I just wanted to say thank you for your simple solution to such an overwhelming problem. I was lost over what to do and after scrolling through all the useless information from Dell, I found your post. I did what you said, it worked, and it saved me. People don't stop often enough to tell someone when they did something good. You did good! And more than 2 years after your original post, your comments are still helping others. Thanks again!