Yesterday I was working with a customer who totally redid vCenter, which included getting rid of vCenter and installing a new one. Once the new vCenter was created he added the old ESXi hosts back to it, but now his VMs wouldn't ping. He tried redoing the VMM domain in the ACI APIC, but he still couldn't get his VMs to communicate. They couldn't ping the default gateway, nothing.
After doing some troubleshooting we realized he had assigned the wrong physical uplinks to his vnics which were assigned to the ACI DVS. It would have been fine, but he happened to be using Cisco UCS with his setup, and the service profile set up had specific physical uplinks with the right nic profile to be used for his VMs. Once we removed and then re-added the physical nics to the proper uplinks everything started working.
So, make sure to double check your service profiles if you experience some connectivity issues.