Suddenly I can't modify any of my Word files. I can look at them but I can't make any changes. Is there any setting in Office 365, Word or Windows 10 that could produce this symptom? If not, what could have caused this? Has anyone successfully solved this problem? (MalwareBytes finds nothing wrong.)

I know there's nothing wrong with the Word files themselves because I use DropBox to automatically sync all Word files to another system. On the other system I can edit the Word files with no problem.