If you see a Connect to Xero button instead of Sync Now, this means the connection is no longer valid.
To re-establish the connection, click Connect to Xero
Follow the prompts to choose the organisation
Reconnect
If you need to reconnect more than one organisation
Simply scroll to the bottom of the Xero connection screen
Click connect # organisations.
Once the connection has been re-established, you can then click Sync Now to update to Calxa.