If clients are lower version than Sep12.1 R2 then you van contact to symantec support.
version is Sep 12.1 ru2 or higher then follow the link
Restoring client-server communications by using a client installation package
http://www.symantec.com/business/support/index?page=content&id=HOWTO81455
See the link for manually replace the file
How to change the sylink.xml file in Symantec Endpoint Protection (SEP) 12.1
#Edit#
How to create a new case in MySupport
http://www.symantec.com/business/support/index?page=content&id=TECH58873
Phone numbers to contact Tech Support:-
Regional Support Telephone Numbers:
Australia: 1300 365510 (+61 2 8220 7111 from outside Australia)
United Kingdom: +44 (0) 870 606 6000
see this one
https://www-secure.symantec.com/connect/forums/where-download-sylink-replacer