What may be an option is to reinstall ESXi using the installation CD. Once the installer shows the available disks, ensure an asterik shows up next to the disk ("* Contains a VMFS partition") before you proceed. At this point you may press "F1" to check the discovered datastore name.
In the next step, select "Install ESXi, preserve VMFS datastore". This will reinstall ESXi but keep the current datastore. You will need to reconfigure any ESXi settings and add the VM's back to the inventory after re-installing, but at least you'll be able to access them again.
André