If you used the DHCP option to configure the IP initially, you will now have to assign as static IP for the FreeNAS server from the WebGUI using any network connected PC. If you assigned a static IP from the outset, this step is unnecessary, although you can use it to check the details.
To do this: Go to Network -> LAN
In IPv4 Configuration, select Static Type, IP address change the final number to 250 from the IP FreeNAS currently is assigned, and Gateway your router's IP address (should already be present).
Click Save at the bottom of the page.
You will then need to reboot, as suggested by the link near the top.
You will then need to login again using the newly assigned static IP given above.
Why not just leave DHCP on to do this automatically? The problem is that FreeNAS IP may change from time to time, which makes accessing it troublesome, since you may have to guess the IP again in the future.