Touch bar disappeared in Macbook Pro 2019
The Touch Bar on Apple's MacBook Pro line of laptops is a capactive touch display not unlike the screen in an iPhone; it's just much longer and thinner.  This means it's susceptible to all of the same ailments that plague every other touch screen display - cracked/broken LCD, touch no longer functioning, blank display, etc.  Some are obvious (i.e. a cracked screen) and others not so obvious; like a blank or non-responsive screen.

In this case, a blank Touch Bar could be either software or hardware.  So, to determine what the issue is, you need to go through a process of elimination.  


Eliminate software as the cause. (See link for steps).  The idea here is to get to a "clean" environment by eliminating as many 3rd party apps, kexts, configurations, etc. that could be causing an issue.  You want to follow these steps so you don't go through the trouble of reinstalling macOS.  There's no point in expending the time, effort and risking data for something that won't work.  Booting into Safe Mode or Recovery Mode has the same benefits of reinstalling without the extra hassle.
It's likely hardware.  There a very good chance that the hardware has failed in some way.  It could be the display itself, the  driver (not software), or even the T1/T2 chip (drives the Touch Bar). Unfortunately, there's no easy DIY way to diagnose it.  Since the entire top case assembly must be disassembled to get to it, the usual fix is just to replace the top case with a new one.   

Bottom line, it needs to go in for service.


Note:  the fingerprint reader (Touch ID sensor) is located adjacent to the display and both are behind a single pane of glass.  It's not uncommon for folks to believe that the Touch Bar must be working because Touch ID still works.  No; they are two completely different components, but since the T1/T2 chip handles security, it's a good bet that the chip is not at fault for the ailing Touch Bar.