This happened to me too. I’ve already done the initial update when I run the sim the first time, and I went to the content manager to download the rest of my contents. Then I leave my PC to let it download itself, by the time I was back it was on my PC lockscreen. It looks like my PC crashed to BIOS and reboots while it was downloading the contents and for some reason it might be corrupted. Because when I run the sim again after that, I was stuck in that “Checking for Updates” screen.
So I’m doing a reinstallation of Windows and the Sim, and I underclock my GPU and set my graphics set to Low so it would be more stable as I download the contents from the manager.