SQL Server 建立连接时出现与网络相关的或特定于实例的错误

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<connectionStrings>
<add name="mycon" connectionString="Data Source=琛哥;database=Blog;uid=sa;pwd=123" providerName="System.Data.SqlClient"/>
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</configuration>在与 SQL Server 建立连接时出现与网络相关的或特定于实例的错误。未找到或无法访问服务器。请验证实例名称是否正确并且 SQL Server 已配置为允许远程连接。 (provider: SQL Network Interfaces, error: 26 - 定位指定的服务器/实例时出错) 
提示的是这个问题，求大神帮忙解答