My Sims 3 World Won't Load

@escapedpolarbear  In your saves folder (Documents\EA\TS3\Saves), you'll find a folder corresponding to each saved game.  You should also see an auto-generated backup for each save, e.g. SunsetValley.sims3 and SunsetValley.sims3.backup.  If you remove the .backup extension from the extra save and rename it (so maybe SunsetValley1.sims3), it will show up in the Main Menu as an option for you to load.  Now would be a good time to copy this file (right-click to see the option) and keep a spare or two somewhere safe, in case something else goes wrong.

When you use "save and quit" to exit your game, sometimes the save function can fail and corrupt your save file.  Then you may not be able to load that world going forward, as you've seen.  For this reason, it's always best to "save as," renaming the file, and then quit without saving.  That way, if the game can't save your progress, it won't overwrite your existing (presumably healthy) save file.  Many of us use "save as" most or all of the time so that we have a number of backup saves to which we can revert if something goes wrong.  I personally keep about half a dozen at any one time, and I delete the older ones when I don't need them anymore.