Odd behavior

ZinnaCookie​  To be clear, you moved the save file to your USB stick, and now your USB doesn't show the save?  How exactly did you move the file?

Separately, uninstalling the game shouldn't have removed your saves; they should still be intact in Documents > Electronic Arts > The Sims 4, unless you happened to have installed the game itself in that exact location the last time around.  Other than that one scenario, this Sims 4 folder isn't deleted during the uninstall process.

I moved the file with copy and paste to the usb stick.

I uninstalled the game twice, first time with the save file still there and the bug still happened, then i uninstalled again with the save file gone, thinking that might help, but I have no idea if that worked as the save file was not on the usb stick :(.

ZinnaCookie​  It's worth searching your computer for the save, just to make sure there isn't a copy hiding somewhere.  Open a File Explorer window, click This PC in the left panel, and enter Slot_ in the search field in the upper-right corner.  The search may take a while to finish; when it's done, review the modified dates on the files it found to see whether your save is there.

You can also check onedrive.com if OneDrive has been active on your computer.  Even if you never use it, OneDrive could have synced your files in the background without your noticing.

As far as corrupt saves go, the files themselves are typically intact, just not readable by the game, or if they're readable, they can't be loaded properly.  The file itself shouldn't just vanish.