i have a laptop with a broken screen, so i took the screen part off and plugged in a computer monitor. Everything was fine at first but now something happened and it shut off and now the only way i can use my computer is on safe mode which isnt ok because i dont have sound. so how do i change my main monitor on my computer without seeing the main computer screen?

Operating systems? Most likely just right click on the desktop then properties and select the monitor and set as default.

hold Fn and press f5 is the normal way to switch to your external monitor.
there should be according symbols on your keyboard