Not sure if this is relevant or not, but the latest update mentioned fixing instances of disappearing sims. Whether they mean sims becoming invisible or just vanishing like the problem you have. Hope you sort it out though.
Update: 9/24/14
Bug Fixes / Issues:
Fixed some issues related to error codes 102 and 122--Fixed an issue that prevents loading an auto-saved game directly from the "Load Game" menu.
Fixed some instances of disappearing Sims.