Using external display is a great thing. I prefer to use my big external screen rotated by 90ยบ: quite useful for long docs.
The problem comes along when you remove the cable while the mac is sleeping: sometimes the system remembers that the screen is rotated but this no more synced with the monitor. It causes that almost half of the screen is not used. Moreover you cannot reset it from the system preferences.
In this case it’s better to drop all the remembered settings for the displays.
You can do it by deleting the “com.apple.windowserver[long text here]” files in “~/Library/Preferences”.
After you deleted the files just restart without saving the window states.