try to log in as root locally, I needed to press ctrl-alt-f7 to get to a log in prompt.
Once logged in as root, I used the beadm command.
Try beadm list and activate beadm activate <old-version>.
I followed this process and then got my FreeNAS server to boot again.
From the GUI I was able to create myself a config backup.
I then burned a new FreeNAS CD and used that to install a fresh new install on my USB drive and then once that booted I used the GUI to restore my config.