chek the network connectivity or Try ro replace sylink.xml either use sylinkdrop.exe or to replace sylink login to sepmanager
select the group right click export communication settings or go to SEPM server (C:\Program Files\Symantec\Symantec Endpoint Protection Manager\data\outbox\agent) put this file on a shared drive or usb as it needs to be replaced
go to client computer
open sep client change setting client management and disable tamper protection
then go to start run type smc -stop
go to C:\Documents and Settings\All Users\Application Data\Symantec\Symantec Endpoint Protection\12.1.671.4971.105\Data\Config
and replace file exported above in here simply paste it
go to start run and type smc -start
check if the client comes on line
note there is space between smc and stop /start