The calendar says "not connected"

This happens when access to Google has been revoked: someone removed it in their Google account, the account changed password, or the access expired.

Go to Settings → Calendar and connect again. It takes three clicks.

Nothing is lost. The bookings already in the calendar stay put. But while the connection is gone, the assistant cannot see free times — so it takes a message instead of booking. That is why it is worth fixing quickly.