In my case this 'suddenly' occurred after a Windows 7 Microsoft restore to previous Restore Point
For reasons best know to Mircrosoft the restore deleted almost all the library.properties files from the installed libraries in the sketchdir /libraries directory, (along with .exes from Arduino15 installed packages)
So don't believe it when Microsoft says restoring to a previous restore point will not effect you 'data' files.
The solution was to restore library.properties from my backup.
Or just use the library manager to 'uninstall' the library and then re-install it.