Check cable connections too. Also be sure the monitor is actually turned on.
I have a dual monitor set up at work. One monitor is hooked up to the computer using and AV to AV cord and the other one is AV to DisplayPort. This set up had been working great for me. Yesterday I took a sick day and came back to find the monitor hooked up to the DisplayPort was not longer working. My coworker informed me that while I was gone a lady who works here (who is completely computer illiterate) set up shop at my desk using my computer. I've tried looking at my settings, but my computer will no longer detect any monitors plugged into the DisplayPort. Any ideas on how to get my second screen back?

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