Typically, you will need to press the Function (Fn) key and one of the functions keys like F8 or F10. If the problem occurs on a laptop, use the function key on the keyboard to increase the display brightness. (You may need to check the manufacturer support website for specific details.) It's also a good idea to check the monitor is using the correct input.
When you connect to an external monitor, open the control panel on the display to increase the brightness to resolve the black screen problem. In the case that you recently updated the UEFI (Unified Extensible Firmware Interface) or BIOS (Basic Input/Output System), this can sometimes disable the discrete graphics card, which you can only enable again from the motherboard firmware.
If the device has both a discrete and integrated graphics card, you can try switching to the integrated video card to determine whether the discrete card is the problem.
