数据库连接错误

可能的解决方案是检查连接字符串是否正确，确保数据库可用并已启动。另外，也可以尝试增加连接超时时间，以便程序能够在连接失败后重新尝试连接。以下是示例代码：

<connectionStrings>
  <add name="MyConnectionString" connectionString="Data Source=myserver;Initial Catalog=mydatabase;User ID=myusername;Password=mypassword;Connect Timeout=30" providerName="System.Data.SqlClient"/>
</connectionStrings>

<appender name="AdoNetAppender" type="log4net.Appender.AdoNetAppender">
  <connectionType value="System.Data.SqlClient.SqlConnection, System.Data, Version=1.0.3300.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
  <connectionStringName value="MyConnectionString" />
  <bufferSize value="100" />
  <commandText value="INSERT INTO Log (Date, Thread, Level, Logger, Message) VALUES (@log_date, @thread, @log_level, @logger, @message)" />
  <parameter>
    <parameterName value="@log_date" />
    <dbType value="DateTime" />
    <layout type="log4net.Layout.RawTimeStampLayout" />
  </parameter>
  <parameter>
    <parameterName value="@thread" />
    <dbType value="String" />
    <size value="255" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%thread" />
    </layout>
  </parameter>
  <parameter>
    <parameterName value="@log_level" />
    <dbType value="String" />
    <size value="50" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%level" />
    </layout>
  </parameter>
  <parameter>
    <parameterName value="@logger" />
    <dbType value="String" />
    <size value="255" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%logger" />
    </layout>
  </parameter>
  <parameter>
    <parameterName value="@message" />
    <dbType value="String" />
    <size value="4000" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%message" />
    </layout>
  </parameter>
</appender>