why?
few options:
1. firewall
2. network connectivity
3. server is not running
follow this steps:
1. ping the server
2. check for firewall existance
3. depending on your os:
1. windows - check that "NetXMS Core" service is running
2. linux - run "ps -ef | grep netxmsd" and see if the server is running
4. check the the version of the server matches the version of the client.
*** after an upgrade there is a chance that the server will not run since the DB has to be upgraded.