Either software or hardware.
Let's check the hardware portion. On the majority of laptops there is a function command to increase and decrease the LCD Brightness level. Try holding the function key (usually on the bottom left or bottom right hand side of the keyboard) and looking at your F1- F12 key's. One or two of the key's should be marked with Star icon with an arrow pointing up or down. If the screen get's lighter or dimmer then we know it's not the hardware.
On the software portion it all depends on your Display Adapter and LCD Manufacturer for the laptop. There should be a utility in your control panel that determines your power settings. If there isn't a proprietary application per the drivers installed for the display adapter, try the Windows power utility. What you are going to be looking for is a field for "When charging" or "While plugged in".