it could mean a couple things.
1) if the computer doesn't automatically output video to an external screen, the port may appear dead. In the bios there may be a setting controlling this. So I think you might still have a shot at repairing it with a new screen.
2) if you don't know more, chances are that the video card is shot. so you are working against odds here.
3) make sure the external is plugged in when you start and then make sure you are using the laptop's swap button to switch over to the external. Probably already did this, but it's gotta be said.
4) there's a chance that you can replace the screen and have it work, but that will mean that there is voltage going to the screen and the screen isn't responding to it. If you don't mind blowing the board, get a voltage meter and check the backlight connection at the inverter and the screen's input ribbon cable for voltage. It's rare that you would get neither backlight nor a dim image if the screen is the problem though. If you don't have a volt reading from the inverter, either your inverter is toast or your video card is the problem (or a wire got pinched and cut). If you don't see any picture (check for a really dim image) than either the screen image board is shot or the video card is dead (or the cable is damaged or lose).
Check all connections, check voltage, and check to make sure you are following the instructions for getting the external monitor to show up. Maybe some of that will tell you something. Good luck, sorry if this is really redundant and basic though, I don't know what you've done or know how to do.