[ubuntu] Highest screen resolution went missing.

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http://ubuntuforums.org – I have Viewsonic VX2253MH-LED monitor. It worked with Ubuntu on 1920x1080 resolution for almost 1 year. But now suddenly the resolution changed and 1920x1080 option is not available in the list of resolution. Can anyone help me in bringing back the 1920x1080 resolution? Thanks in advance (Hardware)