Diagnostic suggestions:

Start Word in Safe Mode. Then open a problem document from within Word; File > Open. Note, save it to the hard drive before opening it.

What happens? Note, the normal style's font in Safe mode will be Calibri. To open Word in Safe mode:

· Hold Windows key + R.
· Copy and paste, or type the following command in the Open box, and then press Enter
· Note: There is a space between Winword and '/'.
· (The above may not work.)
· Or, try holding down the Ctrl key when starting Word.
· Then release the Ctrl key.
A window will pop up asking if you want to start Word in Safe Mode. Click on Yes and let Word open in Safe Mode. Use Ctrl+N and create a new document. Does this work?
Note that safe mode is not a way to operate Word on a daily basis. It is a diagnostic.
How to Start Microsoft Word in Safe Mode