Hi Hussar 54.
Please describe exactly what happens. Does the screen go dark after you log in or before? If before, exactly when in the startup process does it go dark? Before the loading Windows animation or after it?
As others have said, a key test is to start it up with an external monitor. If it works fine, then you have a hardware problem with the screen or the backlight inverter. Does the image get easier to see in the dark or easier to see when it gets lighter?