Don’t worry: your computer is doing exactly what it should — when you first boot up, your computer is designed to send out a signal on all of its connected monitors during the POST and startup process so you can troubleshoot if there’s a problem with a monitor. What happens when Windows itself finally loads is that your computer’s monitor management drivers kick in, and by default they’re only designed to show your screen on the DEFAULT display, which in your case would be the LCD screen built into your laptop.

To get the external display working again, you have to enable it in your monitor settings. If you have an ATI or NVIDIA logo in the bottom right-hand corner of your screen (in the system tray) you can click it to open up the display driver…and then you’ll look inside for the monitor configuration. If you want both monitors to be the same, you’ll want to choose “mirror” or “clone” or whatever the equivalent is in your driver config. If you’d prefer to use the monitor as a separate display (as an extra workspace) you can simply enable it as a second display.

If you don’t have any obvious display management software installed in the system tray, you can use the built-in Windows driver to accomplish this. Right-click anywhere on the desktop that’s not being used by another application, choose “Properties” and then click on the “Settings” tab, while your computer is connected to the external display. You’ll see two squares with 1 and 2 in the window at the top. Click on the one that says “2″ and then below you can check the box that says “extend my desktop onto this display”. Once you click Apply or OK, it should enable that monitor…you should then adjust the resolution using the slider bar. You can also change the position of the monitors in relation to each other by sliding the boxes labeled 1 and 2 around, so that they more accurately reflect your actual workspace.