All of the sudden one day my laptops display got very dark and is hard to read. If I plug in an exturnal monitor it works fine but the scrren is still dark. Please, Is there any way to fix it.

If you mean the laptop screen is dark but an external monitor is okay, either the laptop voltage inverter for the backlight in the laptop LCD display screen is burnt out, or the backlight for the LCD display is burnt out. If the former, the display will be very very dark all the time; if the latter you may be able to make out the beginning of your boot, but then it is so black you can't make out the later stages of Windows loading and your desktop screen (the voltage invertor shuts down when a defective backlight is detected). A failing backlight often gives the screen a pinkish hue just before it fails.
The voltage invertor is relatively easy to replace yourself, but replacing the backlight is a finicky job better left to someone who has done it before - your local laptop repair place probably has someone who knows how to do it right. The official name for the backlight is a CCFL - cold cathode flourescent lamp. The voltage inverter produces high voltage for that lamp.
Your only other alternatives are to get a new LCD display assembly (quite expensive), or for some your can get a rebuilt one for much less money (the CCFL has been replaced properly; possibly $150).

If you mean both the LCD laptop and external monitor screen are dark, you've got big problems. That would mean the video has major problems, and/or the power supply when on the AC adapter has major problems, or ???.

the laptop screen is dark but an external monitor is okay