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The machine was originally connected with two monitors and worked perfectly fine. One of my colleagues used one monitor for his laptop without disconnecting either of these two monitors from the ubuntu machine. After he disconnected the laptop, the ubuntu machine could not recognize one of the monitors. How can I go back to using two monitors?

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I often have this issue when unplugging DVI monitors. Simply reboot the machine and/or power cycle the non-working monitor. This has always fixed it for me.

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I did reboot the system several times, but I still can not get the two monitors. Any other suggestions? – Fan Chen May 23 '12 at 17:48
Have you power-cycled the monitor though? You may also want to check that the monitor didn't get switched to the wrong input i.e. DVI vs VGA. The way you change is different for each monitor. – reverendj1 May 23 '12 at 17:49
What did you mean power-cycled? – Fan Chen May 23 '12 at 17:51
Just turn it on and off again. – reverendj1 May 23 '12 at 17:59

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