ESX 3.0 Host not responding ?

It might be because the host daemon or the virtualcenter agent has crashed. Follow the instructions in the article to check or restart them.

Check that vpxa is running by logging into the console and running the following command

ps -ef | grep -i vxpa

If you get a response similar to the one below it is probably working.
[root@hostname init.d]# ps -ef |  grep -i vpxa
root      1341     1  0 Jul07 ?        00:00:00 /bin/sh /usr/bin/vmware-watchdog -s vpxa -u 30 -q 5 /usr/sbin/vpxa
root      1361  1341  3 Jul07 ?        02:31:06 /usr/lib/vmware/vpx/vpxa
root      9093  9028  0 00:38 pts/0    00:00:00 grep -i vpxa

The results show us that there are 3 processes with the "vxpa" in them. It even lists our query.
If it was not running, more than likely you would just get your own query back. Something like this.
root      9093  9028  0 00:38 pts/0    00:00:00 grep -i vpxa

If it is not running then VirtualCenter cannot connect to the host, and you need to start the agent.
To do this type "service vmware-vpxa start"

If it is running and you want to restart it, you can grab the lowest PID number which is 1341 in this case and run a kill -HUP 1341 or a service vmware-vpxa restart

Check that hostd is running by logging into the console and running the following command

ps -ef | grep -f hostd

If you get a response similar to the one below then it is probably working.
[root@hostname init.d]# ps -ef | grep hostd
root      1244     1  0 Jul07 ?        00:00:00 /bin/sh /usr/bin/vmware-watchdog -s hostd -u 60 -q 5 -c /usr/sbin/hostd-support /usr/sbin/vmware-hostd -u -a
root      1253  1244  6 Jul07 ?        05:00:18 /usr/lib/vmware/hostd/vmware-hostd /etc/vmware/hostd/config.xml -u -a
root      9175  9028  0 00:45 pts/0    00:00:00 grep hostd

The results show us that there are 3 processes with the "hostd" in them. It even lists our query.

If it is running and you want to restart it, you can grab the lowest PID number which is 1341 in this case and run a kill -HUP 1244

If it was not running, more than likely you would just get your own query back. Something like this.
root      9175  9028  0 00:45 pts/0    00:00:00 grep hostd