If the host is disconnected from vCenter Server, you won't have any options to do actions like vMotion, which basically means you will have downtime unless you are able to resolve the disconnection issue online. If you cannot resolve the issue, you may do a clean shutdown of the VM from within the OS itself. After shutdown further options depend upon whether you can still connect to the host directly. In any case, if you somehow unregister the VM and re-register it, you may need to ensure that other applications which rely on the VM's ID (e.g. image based backup applications) are aware of the VM's new ID.
André