Are you sure the screen is actually completely black and not just un-lit? If it's an old laptop model with LCD "lamps" to provide backlight, they might have expired, broken down or have just been deactivated.
There should be a FN button combo to disable/enable screen backlight, so try that first. If it doesn't work, you can assume the worst.
If you're absolutely sure the laptop is up and running (e.g. You hear Windows' distinctive bootup sound, or you can connect it to another display/TV/monitor and it works), then it's most likely the screen itself which physically needs repairs.