Reconnect first
Open Settings, then Devices. A connection that needs attention says so on that screen, and Reconnect is what gets it back. Most stopped syncs end here.
Why it expires
The permission you gave your provider has a life of its own and ends on their schedule, not ours. We cannot renew it on your behalf, which is why the screen asks you to.
After reconnecting
Recent nights backfill on the next sync. A gap in the middle of your history fills in once the provider hands those days over.