Breathe: nothing below deletes any of your mods or your save; only step 4 erases a file, a cache the game recreates. Here is the triage volunteers do by hand on Sims Discords, in order — from the cheapest step to the most expensive. Stop as soon as a step explains your problem: going further would only send you looking in the wrong place.
Before touching anything, copy your Mods folder somewhere else — the desktop, an external drive, anywhere. It takes two minutes and it makes every step below risk-free.
This is a common way everything breaks at once, and a quick one to fix: the game disables mods with every patch. In game: Options → Other → tick "Enable Custom Content and Mods" and "Script Mods Allowed". Then quit the game completely and reopen it: the change only takes effect on restart.
It looks too simple to be the cause — and yet it's a step often skipped, even by experienced helpers, who then recommend a full reinstall.
Three mechanical traps, and the game stays silent on all three. An archive (.zip, .rar) left as is never gets read: the game doesn't look inside, you have to extract it. A .ts4script file only works one subfolder deep — any deeper and the game doesn't see it (.package files can go deeper). And the Resource.cfg file at the root of Mods: if it was renamed or emptied, no .package loads at all; delete it and the game recreates a healthy one on the next launch.
A telltale sign: the game calls a file "corrupt" and it's a different file every time. The cloud kept the contents online and left only an empty stub on disk. Don't delete anything: Windows/OneDrive → right-click Mods → "Always keep on this device". Mac/iCloud → right-click → "Download now", then turn off "Optimise Mac Storage".
While those stubs exist, the 50/50 in step 6 can't work: what you're testing changes from one attempt to the next. Fix this first.
The harmless step, and the only one on this page that deletes anything — and it's something the game rebuilds on its own. Delete localthumbcache.package, at the root of Documents → Electronic Arts → The Sims 4. If the EA app is misbehaving (crashing on startup, empty store), clear its cache too: in the EA app, Help → App Recovery → Clear Cache.
A game that freezes is not a game that crashes: it's a different problem, and the report in the next step won't help you — a freeze writes nothing. Stuck on the loading screen, in the Build Mode search bar, or every few seconds: three different causes, each with the cheapest step first. Game frozen: loading screen, Build Mode search, or repeated.
At the root of Documents → Electronic Arts → The Sims 4, a lastException file often names the mod at fault — not always the right one, and a freeze or a hard crash writes nothing at all. Where it is, how to open it, how to spot a name in it, and the three cases where it blames nobody: lastException: read the game's report, find the mod it names.
The last resort, and it should stay last: only start it if nothing above explained your problem. Begin with the no-mods test — rename your Mods folder to Mods_off and launch the game. If it runs fine, a mod is at fault; if it still breaks, it isn't a mod. The whole method, launch by launch, the notes to keep and the case where no single file is at fault: The 50/50 test: find the guilty mod without deleting a thing.
During the test, bald Sims or missing furniture are normal: don't save, and everything comes back once the files are put back. If that was your problem to begin with, it has its own page — Bald or naked Sims, missing CC after an update: don't save.
It deals with what breaks your game, not with what makes your save annoying: a mod with questionable gameplay choices isn't a broken mod. It also can't help with packs you paid for that have gone missing — that's EA support — or with a copy of the game that didn't come from EA. And if everything above is in order, the cause is elsewhere: this guide won't invent it.
This guide is free and will stay free.