Relationships Have Completely Disappeared

OMG almost all of my sims have lost all of their relationships with other sims! To the point where they don't even have any other Sims appear in their relationship menu besides those in their household! So many hours of world building COMPLETELY GONE. And I have NEVER used mods or CC so they are not the culprit. Even if I load a previous save from a week ago, my Sims will still lose their relationships after awhile of me playing. I've cheated SOME of those relationships using the ModifyRelationship cheat code, but that was only for SOME of the relationships. PLEASE, PLEASE FIX. This is devastating and ruins the entire game and game play.