All my files moved from my local drive to iCloud

Did you just do a system upgrade/update before this happened? It sometimes happens during an upgrade/update that the iCloud Drive is enabled with the Desktop and Documents folder included. There's a window during the upgrade that asks about enabling iCloud Drive and many users have done show unwittingly and end up in the same predicament as you find yourself.

Go to the System/Apple ID/iCloud Preference pane and uncheck iCloud Drive.

In the next window what comes up select Keep a Copy:

I've been trying to "turn off" and get my files back on my iMac. I get through most of the process, but then am stalled at a message: "iCloud needs to finish updating before being turned off." The only options are "Cancel" or "Stop Updating and Turn Off". What do I do?!

Thanks for your reply. Could the updating be a several days-long process? When I tried it, after more than an hour, the progress bar completed, but then reverted back again to the same “needs to finish updating” message with the same “Cancel/Stop updating” options.

You'll have to let it continue uploading till it's finished and then turn it off.