I'm having the same issue. Been happening repeatedly for the past 2-3 years now. Signing in and out of iCloud has worked previously, but it's not a viable long term solution as it requires uploading many GB of files, and takes many hours each time. I have gone as long as 6 months with no issues before, but I've had this issue twice in the 2 weeks. The most recently time, I tried Jurjan's advice above (deleting the CloudDocs folder, and then killing the iCloud Service Helper), which seems to have worked (it still took a while as it re-synced GBs of data, but not as long as a full afternoon of re-downloading). With Apple's cloud services (and iCloud in particular), being a key pillar of their business and product strategy, this really needs to be resolved. People like myself may be considering alternative Cloud Drive options. When iCloud works, it's great, and seamless, but having iCloud fall over in the background without notification or clear resolution is not sustainable. It's also not ideal that there have been no official acknowledgement or feedback of the issue from Apple (unless I've missed it)? Hoping this gets picked up and resolved soon.