Make a backup copy of your project file.
Try renaming the file extension of your project to ZIP. For example, rename MyGame.C3P to MyGame.ZIP
Try opening (double-click) this ZIP archive. If you are able to open - extract files to an empty folder and try opening it in C3 ("Open local project folder").
If you can't open the ZIP file, it's probably corrupted. You can try repairing it with WinZip program, but the chances of success are pretty low.