My Sims & Houses keep disappearing.. HELP?!

S042018​   Ok.  If you are using resume game to load a save and are not ending up right where you left off the last time you played, then you've got something going on.  If the game sends you elsewhere that means that the game couldn't load the save and sent you the last good save it could find.  Lacking a good save, the game will send you to the Main Menu or directly into create a new save.  If the save loaded, but could not get into the lot, you end up on the world screen.

So, from what you said, it sounds like a saving issue.  I'm channeling Capt. Obvious here but make sure you actually save before you quit.  I can't count the times that I've hit exit before saving.  One thing I recommend is to use either the Save or Save As button when in live mode rather than using the "Save and Go To (Main Menu/Manage Household) " or Save and Exit buttons.  Also, many players also recommend to only save in live mode on a lot, not at the world screen, at the map screen, in build mode, or in CAS, particularly if you use mods.  You can begin by running a repair from the EA App.  Just use the manage button that is next to the play on the play screen and select repair from the dropdown.  Once done, rename the "The Sims 4" folder so you can create a new user folder when you start the game next.  Then start the game, create a new game and create and place the sim.  Play a while if you want to then save and quit.  Since this issue seems to take hours to show up, go ahead and play a while to test if it still happens.  If it doesn't happen again, you're good to go.  If it does, then your only recourse is to do a full uninstall/reinstall.  If you want to play the saves in the old user folder, just copy the saves folder over to the new folder.  If you are asked to overwrite any files, choose to save both copies.  You'll have to rename those with the duplicate file names probably.

Hope this helps.