@emilybear28
You can delete your mods but doing that might not work and then you'll have to go download them again once you figure out the problem. Just move the mods folder to your desktop or to another folder out of the game path. Then you'll have them at hand when you are ready to start adding them back. You should check all of them though, to make sure you have the latest version.
But since you play with mods, take a look at your settings for any that might affect culling or otherwise might be causing sims to be deleted. If you are using MCCC, that does have options to control culling but basically the main option is to turn culling off altogether. I haven't looked at those options in a long time so I'm not sure what other options are available. I have turned culling off before for a while but currently I just let the game handle it.
In my last save, I had over 700 played sims and culling wasn't a problem. Neither is it a problem in my current save where I have over 400 played sims.
One other way to prevent sims from being culled is to keep all your played sims under the My Households tab in Manage Households. You need to leave them marked as played as well. Sims marked as unplayed under either tab will still age if you have aging on for unplayed sims in Game Options. If you allow aging for played sims, then to keep non active households from aging, you need to set the dropdown to active household only. Otherwise, all your played sims will continue to age, and some may die before you get back to them if you are a rotational player.
Hope this helps.