This is one of those short posts which is kind of like a digital post-it note for me, which might also help you out as well.
Every now and then, the ESC “key” would disappear from the Touch Bar of my MacBook Pro. A bit of googling brought out the solution in a StackExchange thread: You have to restart the Touch Bar server. Takes a few seconds and then everything is back to normal again:
sudo pkill TouchBarServer