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We had a recent situation where a virtual appliance (Avamar proxy) was mistakenly deployed configured with the IP address of the gateway. This caused the hosts to be unreachable from vCenter. After finding the offending VM and shutting it down from the console, the ARP tables either had to be cleaned up (you could also allow them to time out but that could take up to 20 minutes)

ESXi 5.1
esxcli network ip neighbor list
vsish -e set /net/tcpip/v4/neighbor del IPADDRESS
New functionality added in 5.5 to remove ARP entries through esxcli

Find a VM by MAC in vCenter with PowerCLI
Get-vm | Select Name, @{N=“Network“;E={$_ | Get-networkAdapter | ? {$_.macaddress -eq“00:50:56:A4:22:F4“}}} |Where {$_.Network-ne “”}

Also you may need to clean up your Windows vCenter server from the command prompt
arp -a
arp -d IPADDRESS
Or,
netsh interface ip delete arpcache
Also a good idea to:
ipconfig /flushdns