One of my Sims died while I was offline

This page has advice for bringing ghost sims back to life, with and without cheats: https://www.carls-sims-4-guide.com/death/ghosts.php

If you don't use MCCC, there are other ways to do it explained through that link.

With testing cheats enabled, you can shift click on the ghost sim to add them to your household, then use cheats to bring them back to life.