In many of scenario's I always received one question, how to identify weather the host is in not responding state or disconnect. we need to understand the difference before move onto their troubleshooting. let's understand the difference first if esxi host disconnected or not responding in vCenter.
Not responding state
Host become greyed out and showing "not responding state". Now we need to understand the internal and external factors that causing this issue . in most of the scenario, host heartbeat is no longer available in vCenter. some of the possible reasons are :
network connectivity issue between host and vCenter ( this include ports mapping, routing , cable and firewall associated issues)
hostd is not running successfully on the host.
Vpxa is not running successfully on host
host has failed
The host which is not responding can be back to normal however if disconnected then can be back after adding it again into the vCenter. Check above parameters & services in case of not responding state and understand more through logs. maximum issues got resolved after restart the services or fix network & firewall issues.
Disconnected state
A disconnected host is the one that has been explicitly disconnected by the user, or the license on the host has expired. Disconnected hosts also require the user to manually reconnect the host in the vCenter. Here are some reasons that leads the disconnection issues.
A user right-clicks the host and selects Disconnect.
A user right-clicks a host that is listed as Not Responding and clicks Connect and that task fails.
The host license expires.
In such scenarios, host will remain in the vCenter inventory however does not get any update moreover his health will not reflect any status in monitoring tool. moreover in some of the cases, HA not trigger because host not communicating with vCenter that cause their VM's not migrate onto another host. possible solution, register VM's again on another host and Power-On or once host reconnected, it will be available for failover.
Conclusion
Above is the difference between disconnected and not responding. Do not restart the host immediately during these scenario's. most of the time, above suggested solutions work however you can understand the behavior more from the host logs and if require open a case with VMware for further troubleshooting. For more information on logs study, please refer article https://www.techmajestro.com/post/esxi-psod-error-local-debug-mode and https://www.techmajestro.com/post/esxi-fail-with-psod-no-heartbeat