Sorry for the problem.
First create a backup of your file, even if it doesn't work correctly.
Then, check if you use Sweet Home 3D 1.4, because there was a small bug about saving wrong SH3D files and I fixed it again in that version (see https://sourceforge.net/tracker/index.php?func=detail&aid=1910831&group_id=152568&atid=784665 ).
This bug fix won't repair your file, but at least it shouldn't happen anymore.

I didn't find yet what could cause this problem. It's maybe a software that cleans temp files ?

I can try to restore a part of your file. Please send it to [url=mailto:[email protected]][email protected][/url]