This is a very typical problem and usually happens when new Windows operating systems are released. I have tried changing the Power Settings (via Control Panel) so that the adaptive brightness is turned off, but that doesn't seem to make any difference. It also seems to happen immediately as soon as I login to Windows. The screen goes dim whether or not I have the laptop plugged into the power outlet, or if it's running on battery. I've tried adjusting the screen brightness manually on the laptop by pressing the FN (function) key + up arrow key, but the screen brightness won't say - it keeps going dark automatically on its own.
I just installed Windows 10 on my laptop, but I'm having a problem with the screen fading so dark I can hardly see it.