The files came back when I restarted the program.
I was copying files from my Wordpress parent theme's folder to my child theme folder.
I had just created a new folder into my child theme called "sections". Then I tried to move some files into that folder from the main folder of my child theme. When I drag and dropped one file onto the "sections" folder, everything else disappeared but the file I was trying to move to the folder. Now I have only one file visible under my child theme. Even the "sections" folder disappeared.
But when I go to my website, all the changes I have done in my child theme files are still existing, so it seems like the files are still somewhere but just not visible. Also when I search for the files, it finds them and shows that they are in the child theme directory.
How can I get the files and the folder visible again?