Error message I am shown

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: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

public class commonfunction
    {
        #region Variable Declaration

        SqlConnection _connection;
        SqlCommand _command;
        SqlDataAdapter _Adapter;
        DataSet _DS;
        int _count;

        #endregion

        #region Get DataSet

        public DataSet GetDataSet(string _query)
        {

            try
            {
                _DS = new DataSet();
                _connection = new SqlConnection(ConfigurationManager.ConnectionStrings["MyConnection"].ConnectionString);
                _command = new SqlCommand(_query, _connection);
                _Adapter = new SqlDataAdapter(_command);
                _Adapter.Fill(_DS);
                return _DS;

            }
            catch (Exception)
            {

                throw;

            }
            finally
            {
                _DS.Dispose();
            }

        }
        #endregion

        #region Execute Query

        public int ExecuteQuery(string _query)
        {
            try
            {
                _connection =  new SqlConnection(ConfigurationManager.ConnectionStrings["MyConnection"].ConnectionString);
                _connection.Open();
                _command = new SqlCommand(_query, _connection);
                _count = _command.ExecuteNonQuery();
                _connection.Close();
                return _count;
            }
            catch (Exception)
            {

                throw;
            }
            finally
            {
                _connection.Close();
            }

        }

        #endregion
    }