Suddenly, I cannot open docx or xlsx files from windows 10 file explorer but I can open doc files

edit

As I said, I successfully set the LibreOffice programs as the default program to use to open doc, docx, xls, and xlsx files. The association hasn't been lost, as far as I can tell. I've checked the default apps by file type in Settings, and it shows LibreOffice Writer as the default for all doc_ filetypes, and LibreOffice Calc as the default for all xls_ filetypes. But docx and xlsx files are no longer successfully opening when I double click on them in Windows 10 File Manager.

Try to reassign non-funsion file types. Select LibreOffice as a program (not save mode).

I don't understand what you mean by that. What are the funsion file types, and how do I select LibreOffice as a "program", rather than "save mode".

Okay, this is weird: I just found that I was able to open both docx and xlsx files with LibreOffice by double clicking the file in File Manager. Yesterday, I just got a "not responding" message when I did that. HOWEVER, as soon as the file opens, File Manager displays the "not responding" message and all I can do is close file manager and then open it again if I want to open a different or additional file. When I close an opened file, File Manager is still hung up. I did not make any changes.

See [EDIT_2:] in my answer.

When I right click on a docx or xlsx file in File Manager, File Manager hangs up. It does not open the menu allowing me to choose a program with which to open the file. Also, I wrote a couple of days ago that I WAS able to open files from File Manager, but that the act of opening them left File Manager "not responding". I've discovered that double-clicking in File Manager to open a file ONLY works until the file has been SAVEd in LibreOffice. (see remainder of comment below)

Once a docx or xlsx file has been SAVEd from LibreOffice (and it prompts me to click on a button specifically to save in the "Excel 2007-2019 Format" instead of using the "ODF Format") it can no longer be opened through File Manager...only through LibreOffice. I realize, now, that the problem exists only in files that I have edited and saved in LibreOffice. However, the problem did not occur until a few weeks after I started using this new desktop computer.

I've tested and found that double clicking or right clicking a file in File Manager AFTER that file has been saved in docx or xlsx format in LibreOffice, cannot be double clicked or right clicked to open in File Manger, EVEN if the file has subsequently been saved in doc or xls format. Even though doc and xls formats use more disk space, I thought I would revert to that format for frequently opened files. But that does not resolve the problem.

I've also found that after xlsx and docx files have been saved as xls or doc, or even ODF, the file extension still displays as xlsx or doc in File Manager, and also in LibreOffice file lists or when the file is opened in LibreOffice. Once the file format is changed, the files can be double clicked or right click opened in File Manager, but File Manager hangs up and says "not responding" after that. You have to close File Manager to open another file.