Laptop screen displays nothing when external monitor isn’t connected

Hello so I was just recently about to sell a laptop I’ve had for a few years now and I just finally unplugged it from my monitors to find out everytime I turn on my laptop with my monitors unplugged the screen only displays solid black? When the hdmi cable is plugged back in the screen is instantly back on, so I know it isn’t an issue with the motherboard inside the laptop because the screen works perfectly fine just only on startup it displays nothing. I’ve tried troubleshooting a bit and nothing really seems to be working if anyone knows any solutions that would be much appreciated thank you.