Sometimes, files can become corrupted, this can be due to a number of issues;

Crashing when saving
HDD/Network failure when saving
Plugins corrupting Save files

It may be possible for you to recover some or all of the file that has become corrupted.

For Windows:
Go to search and type CMD
Navigate to your Cinema 4D Program Folder (Normally program Files\Maxon Cinema 4D RXX)
Now, drag and drop your Cinema 4D.exe into the Terminal Window:

Then press space.
Then paste in "-loadcorruptscenes"


For macOS:
Go to Search and type Terminal
Navigate to your Cinema 4D Application Folder (Normally Applications\Maxon Cinema 4D RXX)
Right-click on the Cinema 4D.app and press Show Contents
Navigate through Contents to MacOS
Drag and drop the Cinema 4D file into the Terminal Window:

Press Space
Then paste in "-loadcorruptscenes" (It should look something like the below, )

Once Cinema 4D had loaded, try to open your file, it may still say that the file is corrupt, but then it will try to load what it can.

This process is a "what you see is what you get" and it is not possible for us to recover more than you already have done.

You can also find a video on this process here. 

If you are unable to complete this process at all, please raise a support ticket.