Why does my external monitor not work?

On my laptop, an Inspiron 1525, I took off the monitor and disconnected it from my laptop. Then I hooked up an external monitor. The reason I disconnected my monitor from my laptop was that the hinges were broken and it wasn't even connected to the laptop except by wires. Now when I start the computer up I can see the screen that says I can press F12 etc to go to set up or BIOS etc. Then I see the loading bar when windows is loading up. After that the monitor doesn't show anything even though I can hear the sound of windows starting up. Do I need to have the original monitor hooked up, or do I have to setup something, or will it just not work? I know that the monitor has worked on my laptop before when the monitor was hooked up. Please help ASAP as I am leaving on a long trip in 2 days. Thanks in advance.