Save Files opening in Notepad
In addition to what @roberta591 wrote.you may have saved the savegame you opened in Notepad. That will have given it a .txt ending and when choosing Load Game you would not see it because it now has the wrong ending. To check for this you need to make Windows show known file extensions. To do this in File Explorer select View and place a check by File name Extensions. Notice that older versions of windows may say Hide known file name extensions in which case you remove the check there.
After doing this do you then see a file called Slot_xxxxxxxx.txt or Slot_xxxxxxxx.save.txt If so rename it to Slot_xxxxxxxx.save The savegame will then appear in when you select Open game in the Sims 4. Notice that you chose to associate '.save files with Notepad will have no influence on how they work in the game.
