If you still have your saved games in your Sims 4\saves folder -
[C:\Users\Username\Documents\Electronic Arts\The Sims 4\saves]
It may be the game isn't reading the save files correctly. Have you tried renaming your saved game file?
To rename your save, remove the version number. - .ver0 or .ver1 by deleting the .ver1 on the end.
For example:
Slot_00000002.save.ver1
Rename it to:
Slot_00000002.save
You may want to give it a different number if you already have a save 2 in your folder. The game should then recognize the save file again.
-- If you have an old auto save file (Slot_00000001.save) it's safe to delete as the game doesn't use it anymore. They have removed the Autosave system.
If the above doesn't help, Crinrict has some very useful tips on saved games disappearing:
Save Games disappearing
https://sims4.crinrict.com/en/2014/09/save-games-disappearing/