Well, at least you've learned how to install mods. :-)
What you can do is this, go in game to a computer (or to the town hall), click on it, and find the NRaas interaction. In there OverWatch. Choose Immediate, then in the list look for "Recover missing Sims". Highlight that line by ticking it, then tick the V button for "ok". Let the game run for a few Sim minutes.
Note that OverWatch is a powerful mod, however it is not magic. It can indeed recover missing Sims if there still is something left to recover, but if all traces of the Sim are gone, there is nothing OverWatch can do.