Save file disappeared

LiaKnow​  It sounds like you've looked everywhere.  I'm not sure why the old save would have been deleted since you used "save as," unless you chose the same name and were asked if you wanted to overwrite the current version.  In that case, the failure to save would explain why there's no save at all anymore.

Just to make sure, you checked OneDrive's recycle bin too, right?

The only other suggestion I have is to run a manual search on your computer.  Search in Windows for Command Prompt and open it, then copy and paste this command:

cd "C:\"

(This tells Command Prompt to run the next search on your entire C drive, so you can substitute the letter of another drive to search it instead afterwards.)  Next, copy and paste this command:

dir /s "TravelDB*"

Each save should have one TravelDB.package file inside, so if your save still exists somewhere, this should find it.  You can also use this search:

dir /s "*ExportDB*"

as each save will have a [world name][world ID]ExportDB.package inside.  In either case, if Command Prompt finds a copy of your save, open the location and retrieve it, then put it where it belongs.

By the way, using "save as" does nothing to reduce save file size.  I'm not sure where that rumor started, but it's demonstrably not true.  What it does is prevent the game from overwriting a good save with a bad one... that is, when the good one doesn't somehow disappear.  But backups are essential when working with Sims 3, regardless of how you're saving.