It can be tricky getting video to the external display when the built-in display is not working.
First, try closing the lid of the laptop, then very slowly, gently, and carefully begin lifting the lid just a bit so you can peer into the gap to see whether the display turns on & lights up. Many times when the display cable goes bad you may be able to open the lid to no more than 45º so you can navigate the screen (best to use an external mouse & keyboard so as not to jostle the laptop which could cause the display to go dark again....try repeating the process).
Try using Clamshell Mode by connecting the power adapter, USB keyboard & mouse, and the external display. After powering on the laptop, close the Display clamshell lid. Video should be displayed on the external screen.
You can try toggling Display mirroring on by pressing Command + Brightness Down. This may not always work on the login screen. You can try logging in blindly. You can press the first character of a macOS user name & press the "Return" key. Then type the password and press the "Return" key to submit the password. If you have more than one user account beginning with the same character, then try the password for the other user account.
If only the backlight is out, then try turning off the lights so the room is dark. You may be able to see a faint image on the screen in order to navigate. Perhaps even shine a bright light at an angle on the screen with a flashlight....I don't find this works so well with the USB-C Macs.
Sometimes a PRAM Reset can help.
Yes, I have had a lot of practice trying to do this while servicing my organization's Macs. Usually one of these tips will work for the 2017 & earlier models (later models are even worse & may not always show everything on the external display).
