If you are referring the the ICMP/SNMP status of the node (signified by a red icon next to the node name), then the most likely culprit is an ACL blocking ICMP and/or SNMP Port 161 communication between your SolarWinds polling engine and the target device.
If you are referring to the guest status of the VM through the Virtualization tab, then you would want to start by testing the credentials being used to poll either the vCenter or ESX(i) host.
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