I had this happend with dynasty game, and it was an early sign of corrupted file. After restarting game, sometimes icons will reappear, but eventually disappear again. Than, some Sims become invisible, than LTW disappeared, then I could have only one active Sim with messed up controls, etc. I lost that file, but restarting game would sometimes help even if you save with no icons in town view, but as I said, it didn't stop there. Only difference is that I couldn't see any icon in map view no matter which Sim in household is active.