SQL Server Error 40 appears when the client cannot connect to the SQL Server instance due to issues such as incorrect server name, disabled TCP/IP protocol, firewall restrictions, inactive SQL services, or improper remote connection settings. Identifying the root cause is essential to restore database access quickly and prevent workflow disruption. Proper configuration checks and network verification steps can resolve SQL Server Error 40 effectively. If database files are damaged or inaccessible, a professional SQL Recovery Tool can repair MDF and NDF files and recover critical data securely.