Intuit can end the connection
Access can be revoked or simply expire. When that happens, Crewfather pauses syncing and shows a notice in the QuickBooks section of Settings, with a button to reconnect.
Nothing is lost while it is paused
Records queue up and are sent after you reconnect. A week of invoices raised while the connection was down is not a week of invoices you have to enter by hand.
Reconnecting
Use the button in the notice, or connect again from Settings, then Integrations. You sign in at Intuit and pick the company, the same as the first time. Syncing resumes.
Checking on it
The QuickBooks section shows the connection state and the time of the last sync, which is the number to look at first. A last-sync time from three days ago is the symptom; the notice above it is usually the cause.
Deleting something in QuickBooks does not stop it
If you delete a Crewfather-created record inside QuickBooks Online, the next sync sends it again, because Crewfather still has it and still believes it belongs there.
To remove something for good, delete it in Crewfather.