just updated to Big Sur 11.1 and all my documents vanished?

Are you referring to documents in your Documents folder? Did you have System Preferences > Your Apple ID > iCloud Drive > Options... > "Desktop & Documents Folders" option enabled BEFORE you upgraded? Did you backup your Mac's system drive BEFORE updating?

FWIW, I just updated my 2018 Mac mini to macOS Big Sur 11.1 and had not experienced any loss of files in my Mac's Documents folder. Note that I do not have the "Desktop & Documents" option enabled.

Yes, I don't use iCloud for this purpose, and not knowing exactly how the OP has configured their system, I am attempting to get them to clarify which. As such, if they confirmed that they were using iCloud, I would have provided them with a similar solution as you are giving me. Regardless, thanks for your comments.

You seem to be using local storage "On my Mac". I think the OP has been using iCloud storage and now needs to check the iCloud Documents and Desktop items you point to. They can then select "Download Now" from the iCloud Drive menu, to repopulate the computer.