Laptop screen black

[Greg]

When I boot up Dell notebook Inspiron 5100, the screen is
black. If I hook up an external monitor, the external
monitor displays fine on boot up but then I can't toggle
back to laptop screen. Reloaded video card driver with no
success. When booting up to notebook screen, the windows
blue logo screen does not appear but it does when booting
up to external screen. I can boot up in safe mode and
notebook screen will display. What do I need to do to fix
it so notebook screen will display when starting. Can't
use notebook without an external monitor hooked up.