Thorgalak, even if it's large, your file contains only zeros i.e. no actual data.

This tip may help you to recover recently opened files:
Look in the sub folders of C:\Users\user\AppData\Roaming\eTeks\Sweet Home 3D\work (where user is a login name) if you can find some files starting by "open" and suffixed by ".sweethome3d". If there's any, it will contain a copy of the document you recently opened. Copy the most recent file(s), change its extension to .sh3d and try to open it with Sweet Home 3D.

What version of Sweet Home 3D do you use?