@jacobstone1 Babies living in unplayed households often vanish, at least if you don't have mods. It's also not possible to recover a vanished baby without mods—the only thing you can do is revert to a save where the sim was still present.
If you do have mods, or are willing to add a few, let me know.
@jacobstone1 It's possible that NRaas MasterController could help here, but there's no guarantee: it depends on how much of the sim data is left in the game. I'd add ErrorTrap and Overwatch as well, and work with a backup copy of the save. If you use "save as" and rename the save, the original will remain intact under its old name while you experiment on the new copy.
If you're new to using mods, here's a guide to installing them:
https://modthesims.info/wiki.php?title=Game_Help:Installing_Sims_3_Package_Files/Setup_and_Files
With the mods in place, click on the house where the baby lived. (You can also try this on your own household and move the sim over later. It's worth doing each way to see whether the outcome changes.) When you click on the house, select NRaas > MasterController > Add Sim, which will bring up a list of filters. You can click X to dismiss the filters and get a complete list of sims in the game.
If the baby isn't listed, X out the list and go back to Add Sim, this time trying the filter "Type of Sim (Or)" and selecting "Dead." You can also try some of the other filters, e.g. if Homeworld > None is an option, that might work.
It's also worth letting the game run past 3 am, when Overwatch does its cleanup, since one of its tasks is to recover missing sims.
However, if you can't find the baby on any of the lists, and Overwatch doesn't either, it's possible that there isn't enough data left on the sim in-game. If you don't have a backup save to revert to, then you may unfortunately be out of luck.
