Following 5 steps might be a solution for problem .ai files
Launch Adobe Illustrator. Click “File” and select "Open” from the context menu.
Choose the file you want to recover in the file-navigation dialog. Highlight the file but do not open it.
Press “Ctrl-Alt-Shift” on a Windows PC, or press the Command-Option-Shift” keys simultaneously on a Mac.
Click “Open” while holding down the keys. A blank window will open. Close this window.
Navigate to the directory containing your file with the work you want to recover. Open it and look for a copy of the original filename preceded by an underscore (“_”) character. This is your recovered file ready for editing.