HP laptop screen stays black when I turn it on

Plug it into a TV or monitor over HDMI first. If you get a picture there, the board is fine and it's the screen, its cable or the backlight. If the external screen is black too, it's more likely RAM or the board. Also watch the caps lock key while it's powering on. If it blinks in a pattern, that's HP's error code, and you can count the blinks and look the pattern up for your model. Long blinks then short ones often point at memory, and reseating the RAM fixes it more often than you'd think.