Hi Rama,
NAC agent discovery is failing. NAC agent discovers the server several ways based on the type of NAC appliance deployment method. It uses gateway, it uses last known good server, It uses Discovery host field. If you right click on the NAC agent icon in system tray you may see a field point to NAC server.
If you are using FQDN, DNS may be failing, or there is no connectivity between the endpoint and the Server. Please make sure you open the traffic for NAC server at the port level.
Please call up TAC if you have application support.
Fact of the matter is NAC agent is a EOL product. Last date of application support is July 31, 2018.
More than that, think about moving to ISE + Anyconnect that is the next gen posture solution.
Here is the EOL notice
https://www.cisco.com/c/en/us/td/docs/security/nac/appliance/eol/NAC-Agent-4-x-EOL.html
Thanks
Krishnan