If you use Sweet Home 3D under Windows, check in the subfolders of C:\Users\user\AppData\Roaming\eTeks\Sweet Home 3D\work (where user is your login) if there are some files starting by open and ending with .sweethome3d. If such files exist, copy the most recent one, change its extension by .sh3d and try to open it with Sweet Home 3D.
If you're under Mac OS X, you should rather search in the subfolder Library/Application Support eTeks/Sweet Home 3D/work of your user folder and if you're under Linux, in the subfolder .eteks/sweethome3d/work of your user folder.
If this didn't help, [url=mailto:[email protected]]send me[/url] your file, I'll check if I can recover something.