A “not responding” are displayed after all esxi hypervisor shown in vsphere client which linked to the vcenter. Right click any of the esxi and select “Connect” will bring it back. But after a short while, no more than 90 secs, it will be back to not responding status.
1 In vShpere client, open menu “Administration” — “vCenter Server Settings”, click “Runtime Settings” from the left panel, make sure that all IP and server name are correct. If not, re-enter it. Close and reopen your vSphere client — This is important.
2 Try to ping from your vCenter, not the vSphere client, to each of esxi hypervisors.
3 If you are using Windows Server 2008/R2 or any OS with firewall for vCenter, you need to add or change some firewall rules:
Enable the “File and Printer Sharing (Echo Request – ICMPv4-In)”
Enable the “File and Printer Sharing (Echo Request – ICMPv6-In)”
Add a rule to allow TCP 903 income for ESXi heartbeat.
Add a rule to allow UDP 902 income for ESXi heartbeat.
4 Try to reconnect all ESXi hypervisors.
If the problem still exists, you have to google it again.