If both the web interface and the Diagnostic Tool are not reachable.
First, don't panic!
Second, check these:
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Make sure your hub's IP address didn't change.
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Ensure that the device is on the same network (subnet) as your hub.
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Check your local network equipment and settings (including Ethernet cable, firewall settings, etc).
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Check the hub's LED and if it's red, unplug the hub for 30 seconds before plugging back.