I didn't change any settings and This PC just disappeared. Now it shows Network instead.
Change "Start Folder Tree At" back to the default "Desktop" if you always want This PC (and other branches) to be visible.
Setting it to anything else will make the chosen branch visible but only when you're below it. If you go to something outside of that chosen branch (as in your screenshot, you're in Network, which doesn't exist under This PC) then the branch will be temporarily replaced by the one that's needed to show your current location.