This may occur if your Xero account is connected to another third party source other than the Salesforce instance with InvoiceThis. When connected to the third party, Xero expires the security token issued to InvoiceThis and thus disconnects.
To prevent this from happening, ensure that the Xero user account used to link up with Salesforce is not being used to connect to any other third party platform.
You can easily reconnect and have a new security token issued. To do this, navigate to the Invoice This Setup/Configuration Tab within Salesforce, selecting “Disconnect” and then reconnecting straight away.