Laptop screen blacks out when I plug in HDMI

Hi! I'm not really that good with computers/laptops so I'm taking the chance with this.

I didn't have this problem until last night when I tried playing a game. I plugged in an HDMI cable to my laptop so I can use two screens (laptop screen and external monitor) then I tried loading the game and moved it to the external monitor when suddenly my laptop screen blacks out.

1. The monitor still displays what was on my laptop screen while plugged in my laptop.
2. The laptop screen is okay whenever I remove the HDMI cable.
3. I can move my mouse and interact with the stuff that's on the external monitor.
4. I can interact what's on my laptop screen (did it on a stream with my friend navigating for me) but I can't see anything on my laptop screen.

I hope someone will be able to help me pinpoint the problem so I can fix it. Pls do ask questions if it will be able to help me with fixing this.

Thank you in advance!