Can you restore to default in Calibre Editor?

Some time ago, I was able to make very basic changes in the texts of books I was reading where there were scanning errors, or other formatting problems. Somewhere along the way I did something that completely changed what I was looking at on the screen.

Looking at Calibre's Use Manual, it seems to be the File Browser that I'm looking for, but I don't see any control that would bring that to the screen.

So, is there a restore to default option anywhere?

Jim