HP laptop screen very dark but an external monitor looks fine

Did it start right after a Windows or driver update? If so, roll the display driver back in Device Manager before you assume hardware. Some Intel drivers have shipped with power saving features that dim the laptop panel hard and leave external monitors alone, which is exactly your symptom. If it was gradual, or it changes when you move the lid, it's more likely the display cable through the hinge. Common on HPs after a few years.

Shine a phone torch at the screen at an angle. Can you faintly see the desktop? Then the backlight has died (or its connector), and that's a repair, no setting will fix it. If it's just dim rather than nearly black, try the brightness keys, then Settings > System > Display and turn off 'change brightness automatically' and content adaptive brightness. Battery saver dims it too. Also worth going into the BIOS (F10 at startup on most HPs): if the screen is dark even there, it's hardware, because Windows isn't running yet.