It could be 3 things:
1) Windows settings:
- Click "Start" and then "Control Panel."
- Click "Power Options."
- Click "Choose when to turn of the display" located in the left-hand sidebar of the Control Panel interface.
- Set "Dim the display" to "Never" for "On battery" and "Plugged in" and then click "Save."
2) Ambient light sensor:
Some latitudes come with an ambient light sensor which automatically changes your brightness depending on ambient light. Toggle it on/off by holding down the Fn key and then pressing the Left arrow key.
3) Brightness turned down.
Latitudes all have adjustable brightness, press the Fn key plus the up/down keys to adjust. Some old latitudes have this setting on the F5/F6 keys instead of up/down. Look for blue icons that looks like lit and unlit suns.
If the above fail, your CCFL (Cold Cathode Florescent Lamp) tube in your screen is possibly going bad. They are $10-$20 on ebay and pretty easy to replace. Just search for "CCFL" on ebay and include the exact latitude model you have. All CCFL tubes use the same power, just the length matters, but people on ebay include model numbers so you can be sure it will be an exact fit with your laptop.
Replacing the CCFL is easy, you will need a phillips screw driver, and possibly a shim/flat-head screw driver, that's it. Just watch a related youtube video by searching for "how to replace ccfl latitude" for a number of good step-by-steps.