972. Lost File Sharing on my network

Wow! For years I have been using file sharing across my network allowing me to share files between my wife's PC and mine. Today it ended! I received 0x80070035 when trying to access the file on her system that I have been for years. I saw many interesting articles about this error code, but only one did the trick. It had me (on her system, where the target file resides) look at the properties of her C: drive which showed NOT SHARED. A simple tab selection (Advanced Sharing) allowed me to check the FOLDERS item to share her C: drive. I originally had only the specific file selected before, but decided I might as well get access to her whole drive, which was really my other PC anyway. It worked immediately. I suppose I could have looked at the specific folder that was previously shared to see if it lost its sharing, but did not. I'm satisfied that sharing c: works. Now, what caused the problem? Who knows? No time to mess with this since it works.