[2020] Sturdy structure collapses

Yep. I've seen it several times when a structure that was teetering on the edge of collapse had something change that pushed it over the edge. Usually the cause is plants. People don't realize that as plants grow, they get heavier.

The way to restore your stuff would be to allow cheats/enable creative mode and grab what you need from there. It’s cm in console to toggle creative mode and u to access the menu. It’s dm for debug mode if you need more time before the next horde, or you could again adjust the settings for no horde.