There might be 2 reason for this issue.
One the existing definitions have become corrupt so you can stop the SMC and Symantec endpoint protection service and delete the definitions from \program files\common files\symantec shared\Virusdefs <dated folders>
Then download the latest rapid release definitions from the internet.
Secondly you say even the clients are not able to connect to the SEPM server so that means..
The port 8014 /80 might be blocked by Windows Firewall or there might be a Sylink.xml file mismatch.
You can replace the sylink.xml file from the server to this client and open the ports in the firewall for them communicate