is your datastore connected to host? is it local or SAN datastore?
if your datastore is connected to host and working fine and you know the VM Folder location on datastore, remove that "Unknown" vm from inventory and then browse datastore and go to vm folder.
find the vmx file for that vm and add/register vm inventory.
without removing "Unknown" vm from inventory you will not able to register vm from datastore.
After that you can power on it.
Thanks,
Haridas