What changes have been made that might affect the connectivity? Is a firewall in between silently dropping the connection as opposed to denying it outright? Any changes on the remote host at all such as Nagios not being an allowed host? It seems more like the former, but some more details will help us troubleshoot anyway.
Then try to run a few NRPE commands to it from the XI server, hopefully, you should see xinetd[nrpe] lines being displayed in the syslog, if you do, please copy out those segments and post them here in code wraps. Thank you!