Chances are, you downloaded a corrupt mod - or way too many mods
Try to verify game integrity, could also be a missing file

引用自 Drake-C13：

Chances are, you downloaded a corrupt mod - or way too many mods
Try to verify game integrity, could also be a missing file

Well I did download a lot of mods, so that sounds like that's it. How many is too many? How do you verify game integrity, anyway?

It is best to download and test each mod individually. Before doing this though, verify the integrity of the game files:
Right-click on UEBS in your library -> Properties -> Local Files tab -> Verify Integrity of Game Files

If the game files are all good, delete all your mods and try them out one at a time. Delete them all here (remember which mods you deleted as you need to unsubscribe from them after):
(Where your Steam folder is here)\Steam\steamapps\workshop\content\616560

If you know some of the mods work, they don't need to be deleted. Only delete the mods that you added just before you couldn't get into the game. Once you've deleted the mod files, go back into the game and unsubscribe from all of the mods that you just deleted:
In the mod workshop, the unsubscribe button is the small check mark to the left of the big green play button (the check mark becomes an 'X' when you hover over it).