@Foxley828 I don't know where your other files could have gone, to be honest. The most likely place is OneDrive, if it's running at all—even when people don't intend to use it, OneDrive can still be active. But that's no help for people who don't use it or who might have overwritten the older files with newer ones.
The way to prevent this is to make backup copies and store them elsewhere, i.e. not on your computer. An external drive works, and if you don't have one, you can set up a free account on one of the filehosting sites (e.g. Dropbox) and upload some content there. The free accounts don't offer enough storage to back up everything, but your most recent saves and Tray items should fit, especially if you zip the folders before uploading them. On the other hand, external drives are very large and very cheap.
The Tray folder holds the data for the files that show up in-game in your library. When you save something to your library, several files are generated, with the file types and number depending on what kind of item it is. That's why I suggested you move one set of files with the same timestamp: that should correspond to one saved file. If you're curious exactly what these files are, please see this:
https://www.carls-sims-4-guide.com/forum/index.php?topic=26535.0