First, make sure it is plugged in. A lot of laptops go into powersave mode when unplugged, and they dim the screens to conserve battery.
Second, try rebooting. Pay special attention to right when it starts (before windows loads). If it is really dim then, it is most likely a hardware problem. If not, you might want to poke around the windows settings.
If rebooting doesn’t fix it (and there’s no hardware brightness control on it that might have accidentally changed), it’s likely the LCD backlight that’s failed (since you can still see the screen, it just isn’t lit up). This can be either very expensive or very cheap to fix, depending on how the LCD is made. For laptops, it’s usually expensive, I’m afraid.
Many backlight issues are not actually the bulb but the backlight inverter.
This is actually a fairly inexpensive repair (compared to other laptop hardware issues and is often within reach of repair by a non tech user. Backlights are like little flourescent tubes, the inverter is a little circuit board often located in the display part of the housing. Open case, undo a couple plugs, and plug in new inverter.
Watch the screen as you open and close the lid slowly. You might find that it comes on at a certain point, which indicates a loose connection or a crack in the ribbon cable that’s routed through the hinge area. Repair could be free - just re-seat everything, or you may need a new inverter as the cable is probably a permanent part of it.