I have been having this problem on and off for months. Up until now the screen has come back after a few seconds in response to a mouse gesture or a key press. Sometimes it happens several times in a row. It seems to happen more often when the MBA is plugged into an external monitor. This morning several times the MBA screen went blank while the external monitor was still alive. I tried force shutdown and restart. I noticed that when it was restarting that the boot progress widget was very faintly visible. It looked like the machine booted fully but with the screen so dim it was completely unusable.
My current hypothesis is that what I saw this morning was different from previous incidents. And may be the result of the MBA sitting on top of another magnetic case. I removed the case with the magnets and restarted and, for now at least, I seem to have a functional system.
I would be very interested to hear from a macOS engineer whether the system puts the screen into a very dim state, rather than an off state, in response to the case being closed. If that is so then the problem that I saw today is more likely to be around handling case open/closed state rather than anything more serious.
It still does not explain why I get the repeated brief shutdowns on other occasions. It looks like it may be two separate problems.
ps. I just got another brief shutdown, only seconds after typing the last character, before hitting the reply button on this page. (****!)