Hello,
Could you let us know what OS is the SEPM server running on?
Is the SQL and SEPM installed on the same server machine?
Any errors in the Application Logs, System Logs under event viewer?
Check this Thread:
System is unresponsive due to SemSvc.exe once Symantec Endpoint Protection Manager is upgraded to 12.1
http://www.symantec.com/docs/TECH168248
Hope that helps!!