Can't get external monitor to work

Hello,
Have an older Toshiba Satellite, Windows Vista Premium, with no screen light, so must use an external monitor. I have it all hooked up, I believe correctly, but the external monitor insists on saying "no signal." Then that even goes away. Can't get the picture to come on. If I use a flashlight, I can see that the desktop is up on the laptop, but it's very difficult to read that way, much less navigate, but it is possible if there is something I have to do on the laptop screen itself to fix the external monitor..
Anyone have any suggestions? Thanks.