On some models there is a key sequence - shift-F5 on mine - that deactivates the laptop screen in favor of the external video. Check your manual for what that might be on your unit. Hit it a few times.
With the external monitor attached, you might try getting into Device Manager (Control Panel/System/Hardware tab) and looking for signs of failure under the "Display adapters" icon and the one labeled "System devices".
Then it's the shop, I'm afraid. Good luck and let me know if you get any more information.
Checking that, everything shows ok...gonna try the keystroke combos...