Help! Sims fiancé moved

@Mayaness  I've never faced this problem myself, but I can think of a couple of different approaches that might work.  The first is possible without cheats: pick a prospective spouse, move that sim into your active household (through friendly interactions), and spam romantic interactions until the two sims see each other as "extremely irresistible."  The new sim should then have the option to ask your original sim to break up with the now-disappeared fiancée.  There will be some dialogue about the former flame being very angry, and perhaps the new sim will have gained an enemy.  But the only real downside is that your sim may now be seen as a cheater.  This can be remedied with the Clean Slate lifetime reward.

I'm not actually sure whether granting your sim the Clean Slate could by itself remove his relationship status, as I don't tend to use that LTR.  But you could try it, if you have the spare points; or you could cheat some extra points and experiment.  (And you could quit without saving if it doesn't work.)  To grant extra points, I believe you have to enable testingcheats while still at the Main Menu.  Bring up the cheats console (crtl-shift-c) and enter "testingcheatsenabled true" without quotes.  Then load your game.  When looking at the lifetime happiness tab, there's this one small blank spot just to the right of the treasure chest icon.  Crtl-click on it as many times as you want to increase your sim's point total.

If you have mods, you can also use them to change relationship status, depending on which ones you have installed.

Other than these options, the only other thing you can do is revert to a previous save from before the family moved out of town.