My files are not opening (the html and CSS files)

I am trying to do the projects of CSS i.e. Responsive Web Design projects.
So, whenever I try to open the project I am working on the HTML and CSS file don’t open (the files in which I write the program).
Here is a Screen Shot of what I am trying to tell :

What can I do?
I cannot proceed without getting this thing solved as, even if I create new pens they have the same issue.

9 posts were merged into an existing topic: Codepen not working