Sims 4 launches but stops responding at the main menu

@stab_crab  Did you boot into safe mode instead of a clean boot?  Safe mode doesn't include an internet connection by default, but a clean boot does.

At this point though, this might be a system issue that requires a more pointed intervention.  Given that you can't make a new admin account, I think it would be worth doing a repair install even if you didn't have any Sims 4 issues.  The repair install is a reinstall of Windows without wiping the system: you can keep your apps and personal data, but you get a clean copy of Windows itself.  Here's a guide:

https://www.tenforums.com/tutorials/16397-repair-install-windows-10-place-upgrade.html

If you get to step 13 and are NOT asked whether you want to keep your data, back out and try again.

Once Windows is reinstalled and you've restarted your computer, try playing again, still with a clean user folder, and let me know how it goes.  Please also let me know whether you can now make a new admin account.