External monitor not recognised- MacBookAir I have the same problem with my new MacBook Air (May 2013) as others have stated with MacBook Pro. All was good until I played around with the resolution. Now the second monitor is not seen at all in the System setting; the monitor's power light is on (The monitor is not faulty as I plugged it into my Windows netbook) MacBook Air, monitor connected using VGA adaptor. I have unplugged everything, re-booted, and spoken to it nicely (and harshly). HELP as I have been using two monitors since 1994 and am lost without both. Thanks in advance for any assistance you can offer.
The first thing I would try is an NVRAM reset, instructions here.

Also close all applications and try pulling these plists 


Library/Preferences/com.apple.windowserver.plist and 
/Users//Library/Preferences/ByHost/com.apple.windowserver.XXXXX.plist


to your desktop and reboot