My 5 year legacy save file is gone

fisseryt​  There's currently a bug with the autosave feature in certain circumstances.  This is the description from the Sims Discord:

Additionally,  we're investigating a issue where manual saves can inadvertently override each other during a very specific flow. If you open a save file, make changes, and exit to the main menu without saving, and then load an autosave - choosing "Save and exit" from that autosave will override your latest save instead of the original one.

Is it possible this is what happened to you?  If so, I can merge your post with the bug report.  Regardless, you can try recovering a backup version of this save; you should have a few that the autosave didn't touch if you've been playing the save for any length of time at all.  Here's how to manually recover a backup:

https://sims4.crinrict.com/eng/2014/09/recover-backup-save-game/