As Captain Obvious would say, the first place to look into is the bin.
If it's not there, then it could be that the Desktop & Document syncing with iCloud has been turned off manually, or not reactivated after a macOS upgrade.
To check that, just visit > System Preferences > Apple ID > iCloud > iCloud Drive Options
If the Desktop & Documents Folders option used to be ticked on but isn't anymore, then just reactivate it
If the Desktop & Documents folders option is ticked on, then check the Bin of other computers connected to the same iCloud account
And you, did you activate Desktop & Document syncing with iCloud on your Mac?
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