I'm not that familiar with openstack running under Ubuntu, most of mine were on RedHat/CentOS and Scientific Linux. I've got similar problems before, here were what I check and find out the problems. Since you mentioned you can't ping the instance, wonder whether the instance was really boot up. If you change to the instances_path directory and into the target instance subdirectory, the console.log for that instance should tell you it's status. If you have consoleauth.log you should be able to find out how you define the vnc host and the port for the VM.

I would agree to the above comments that you would need to paste your nova.conf to verify the VNC settings are proper.

Also you mentioned that you are not able to ping your instances provisioned by openstack. First step to debug would be :

1) Go to the host where the instance is provisioned. 2) Install virt-manager if not already installed. 3) Open the VM and check if the allocated ip is actually assigned to your provisioned instance. 4) If not, try to set a static-IP belonging to your network range, try to ping the interface address of the machine where the DHCP Server(dnsmasq) is running for quantum/nova-network and use tcpdump to analyse till where your request is reaching and see if the network is not connected properly somewhere.