Show error when I login SQL server management.

Show error when I login to the SQL server management. First, I change the server name. Second restart the server/1st stop services the start services. Then I check the server name

SELECT @@servername AS 'ServerName'

using this line the show new change server name. Then I closed the window of SQL server then open it and type the new change server but show error. Please help me.... I use SQL Server 2008 r2. Windows authentication

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SELECT  HOST_NAME() AS 'host_name()',
@@servername AS 'ServerName\InstanceName',
SERVERPROPERTY('servername') AS 'ServerName',
SERVERPROPERTY('machinename') AS 'Windows_Name',
SERVERPROPERTY('ComputerNamePhysicalNetBIOS') AS 'NetBIOS_Name',
SERVERPROPERTY('instanceName') AS 'InstanceName',
SERVERPROPERTY('IsClustered') AS 'IsClustered'

EXEC sp_DROPSERVER 'JAYANTA'
EXEC sp_ADDSERVER 'SIASERVER', 'local'