First of all, THANK YOU for submitting an attached image. That makes determining the problem and solution much easier.

If the computer is now unreachable but it now shows invalid heartbeat (logon and uptime) status then we need to dig a little deeper on those issues. When a heartbeat fails then the previous entries of logged on user  and Uptime should be purged from inventory.

The most common cause of the Computer Unreachable error has to do with the Windows Firewall (or indeed any OS firewall) that prevents Remote Administration. Please refer to a previous forum post on resolving this problem.

Another cause (though not as common) for the Computer Unreachable error is when DNS is misconfigured or is providing a different hostname/address for the computer-in-question. In a DHCP environment this will often be resolved within a few minutes. In some cases, however, the DNS admin may have to modify the A record of the host to resolve to the correct address.

How long has this problem been occuring? Have you restarted your Admin Arsenal console (or the AAbackground service) to see if the problem persists? Did you verify the necessary firewall settings are enabled?

Run nslookup to verify that both your nameserver and Admin Arsenal are using the same IP Address. You can also try to disable the "Verify computer name when looking up addresses" feature in your Preferences (Heartbeat pane).