Display and login issues

Sounds like maybe the built-in display may not be working (possible hardware issue), but macOS has some communication with the built-in display so the built-in display is still the main screen. Since the external extended display is blurred and you are unable to interact with it, that tells me either you are still at the login screen, or there if logged in, then there may be some kind of system prompt or notification on the main screen which needs to be dealt with before macOS allows for any other system interaction (good luck with the latter).

If you are logged in, then try toggling mirroring by pressing Command + F1 (or with a Touchbar Mac press & hold the Fn key, then press Command + F1 while still holding the Fn key). This only works after a user is logged into a macOS user account unfortunately. If you are still at the login screen, then try to log in blindly. If you have user icons for the login screen, then type the first character of a user account followed by pressing the "Return" key which will bring focus to it & provide the password prompt. Enter the password and press the "Return" key to submit the password. If you have some other type of login screen, then you will need to figure out how to enter the user name & password blindly.

You can try closing the lid of the display, then very slowly & gently open the display to less than 45 degrees to see whether the built-in display comes back on (signifies a bad display cable...a known issue with some 2016-2019 models, but later ones possibly may still have it too).

Or put the laptop into Clamshell mode so the video should be forced to the external display. You will need to connect the charger to the laptop as well as a wired USB keyboard & mouse. After powering on the laptop you will need to close the lid of the display...I like to do this very soon after hearing the startup chime in order to have the early boot screen with the Apple displayed on the external display. Sometimes it can be tricky to get Clamshell mode to work.

If it is an Intel Mac, then you can try a PRAM Reset (hold for two chimes if possible) which will reset the brightness settings to default just in case the brightness level got set too low. You can try adjusting the brightness anyway, but many times I find even the brightness adjustment is not allowed until after logging into a macOS user account.

You will need to have the Display Assembly replaced. Unfortunately the 2016 model is now considered "Vintage" so an official repair is still possible as long as the necessary part is still available. Have Apple or an Apple Authorized Service Provider examine the laptop to provide you with a repair estimate.