One of the VM instance's network went down i.e it was neither accessible via ssh nor it could communicate with other machine in LAN.
Access was restored when the instance was restarted.
This log was found in syslog " Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP."
Need to know why this happen and workaround to prevent it in future
Edit: Issue is Communication with the VM Instance was down. It was not accessible nor it could communicate with other machines.