something has happened. usually something bad when permissions go missing that once were good. sorry to say, that the best way to fix it is to try to repair or reset the pc, if that doesn't work, I would clean install windows.
the fact that this happened and you don't know why could be malware, virus or other bugs that are not normal.
you can try a System Restore point to when it worked and hope for the best, they are not that great at restoring in my experience.