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?
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.