Now that you can ping the gateway, check whether you are able to reach the host using the vSphere Client. Then check the DNS settings in the GUI (DNS servers). Does ping or nslookup return the proper IP address?
I sometimes saw that a host refused to resolve DNS names, when it wasn't able to contact the DNS server when it was powered on. If all the settings are ok try rebooting the host to see whether this solves the issue.
André