SQL Server connectivity is the most common issue & below mention errors is also common at time of connectivity issue.
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 – Could not open a connection to SQL Server) (Microsoft SQL Server, Error: 53)
Resolution : Although, There are many reasons of SQL server connectivity issue. But I have setup an small checklist that helps me most of the time to find out the real cause of connectivity issue.
1. Check SQL services are running
2. Check SQL Browser service is running
3. Check remote connections are enabled
4. Network connectivity between database & application servers by TRACERT command
5. Check TCP/IP protocol enabled at SQL server
6. Check talent connectivity – telnet <IP address> <port no on SQL server running>
7. Check UDP port 1434 is open or not on SQL Server
8. Check firewall is running or not Check
9. If firewall running, SQL Server & UDP port must be added in exception in firewall