You boot up your MacBook with an external monitor attached, but you forget to turn on the monitor. You turn on your external display, but your MacBook is already booting. When everything is all booted up, your external monitor is at the wrong resolution — 800 X 600. What do you do?
First of all … you don’t have to reboot your MacBook.
Just go to the Apple menu and select ‘System Preferences …’
Select ‘Displays’ from the ‘System Preferences’ window that pops up
Another window pops up that probably has the name of your external monitor in the title bar
Click on the ‘Detect Displays’ button from that last pop up.
Done. Your external display will get back to its optimum resolution.