Solution
I was in Malaysia when I encountered this problem with my brand new MacBook Pro 13", but didn't have the luxury of going to my local Apple Outlet to fix it, so I stayed up late to call the US and figure out how to fix it. My laptop worked when I initially plugged it into the LCD in my hotel room, but then I forgot it on battery and it went to sleep, to never work again until now. Your MacBook will get confused when this happens and it saves this in cache, so you've got to clear it like this.
1. Turn off your laptop, hold Shift + Control + Option and then Power (Note: it doesn't look like anything is happening but it is, ie.) you won't see anything and Mac will not start)
2. Laptop should still be off, then hit Command + Option + R + Power. Still holding these down when you see the Apple logo appear, press down P. You should hear the Apple chime twice and then you can let go.
3. Shut down your laptop, make sure it's pugged into A/C and your external monitor is connected. Press Power + Shift at the same time, which should start you off in Safe Mode and you should see your external display come on also.
4. Now you can restart your machine normally and it should all work for you.
Note: Remember to have your laptop plugged into AC anytime you connect to an external display and turn your power settings to Never under AC as well. When you disconnect, remove the External plug and wait 10 sec, before shutting down or closing your laptop.