Normal file association problems, but since you didn't provide any system information, nobody can tell you exactly. Sounds like Windows, though. In that case eitehr Extras --> Folder Options or Organize --> Folder- & Search Options is your friend... Delete the AI file association, manually reinstate it the next time you double-click a file...
Sorry for the lack of info. I am using apple operating software.
Select a file with the .ai extension. With the file selected, do a Get Info (Cmd-I) for that file. When the Info window opens, scroll down to the section that says Open with: and use the dropdown to select whatever Illustrator app you have installed. After selecting AI in the dropdown, click the Change All button. Close the Info window.