Assuming you're using Windows 10, File Explorer doesn't do a good job at figuring out how to represent the "new" view of files if you make any significant changes to it. As such, it re-shows itself as "This PC".
Occasionally though, if you run "chkdsk" using the instructions below, minor file system errors can be fixed and File Explorer may run better. It's worth the time to run it.
It seems as though the Windows 10 "upgrade" process may not install all the files correctly, or maybe they're damaged, or maybe something else, but running Chkdsk has fixed quite a few of these minor glitches. Try running Chkdsk in "repair" mode to find any damaged files and correct them. Start by opening a command prompt as an admin. RIGHT click on the Start button, choose "Command Prompt (Admin). (Or type "Command" in the search line, then RIGHT click on the "Command Prompt" listing at the top, choose "Run as admin".) Once there, type chkdsk /f /r /x, leaving the appropriate spaces between the letters as shown, then press the Enter key. You should see a note that it can't be run till a restart is done, type "Y" for yes, then press the Enter key again. Close the command prompt and restart the computer. Chkdsk will run upon restart and will take quite a while to finish.. Don't panic when the progress bar stays at 10% for a long time. It's fairly normal and the Chkdsk process may take a couple of hours.
When it's finished, it will boot to your normal startup/lock screen and you can login normally and then see if things are working correctly again.
Hope this helps.
Instead of closing file explorer, the system keeps changing the screen to "this PC". After an hour or so the folder I am in reverts to "this PC".