Typically C:\Program Files\Zabbix Agent\zabbix_agentd.log (Windows) or /var/log/zabbix/zabbix_agent2.log (Linux).
If the Zabbix agent or server cannot physically reach the MSSQL port, data collection will halt completely. zabbix mssql failed to fetch info data -or no data for 30m-
: Run isql -v YourDSNName from the Zabbix server command line as the zabbix user to see if a raw ODBC connection works. Typically C:\Program Files\Zabbix Agent\zabbix_agentd