Your headline and your text contradict each other. Is the host not accessible or a VM?
If a VM is flagged with "disconnected" or "inaccessible" and is grayed out, this usually indicates problems with the storage or the VMX file.
If the host is shown as disconnected or not responding and grayed out, there is usually a problem with the DNS or with the network connection between the host and vCenter. Sometimes there is also a problem with the management agents on the host.
But without more information, we won't be able to tell you which troubleshooting is best.
Maybe, this KB article will help you: VMware Knowledge Base
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Regards,
Sebastian
VCP6.5-DCV // VCP7-CMA // vSAN 2017 Specialist
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