Hello,
What version of SEP 11.x are you running? What OS is this issue occurying on?
Is this issue ocurrying on 1 machine or all the SEP client machines?
If this is happening on all SEP client machines then was there any changes performed on the SEPM server, like rollback of definitions or Tamper protection turned off?
and if this is happening on only 1 client machine, then
Run a Repair of SEP client from Add / Remove Programs and Reboot the machine, then please update the SEP client with Intelligent Updater and check.
http://www.symantec.com/docs/TECH102606
Hope that helps!!