A laptop screen may go black for various possible reasons: there is a problem with the computer's operating system, the screen brightness is set to a low setting or there is a problem with the display. The external display and the laptop display are two different things. Connecting an external monitor to the laptop will serve as a test to see if the external display is working properly.
If the laptop connects normally to an external monitor, the problem could have to do with the laptop's display. In this case, there may be an issue with the laptop's video adapter.
It's important to make sure the problem isn't stemming from the brightness setting on the laptop's monitor. If a brightness setting is too low, the screen may appear significantly dimmed, in which case using the laptop's brightness toggle can make the screen brighter. Also, the laptop could be in power-saving mode, which may dim the screen or shut off the screen display during inactivity. Power modes can be adjusted and power-saving features can be toggled off, if desired.
If the laptop screen goes black after trying to log in, however, this may be a sign of a more serious problem with the computer's operating system. A file could be corrupted. If restarting the computer doesn't resolve the issue, the operating system may need to be restored. This can be done using the recovery console, which usually is available on the operating system installation disc.