The agent process is not running
A laptop that slept, a reboot where the service was never enabled for boot, or a manual stop. From the other end of a socket that is no longer there, this is indistinguishable from a network failure, which is why the dashboard does not try to guess.
The machine is awake and the link is not
A captive portal, a VPN that took the route, or a firewall that started inspecting outbound WebSockets. The agent keeps dialing and logs a connection problem each time, and that is the difference from a stopped process: a stopped process logs nothing at all.
An update started and never finished
A self-update that left nothing running is its own state rather than an ordinary offline, and it is worth naming separately because the repair is not the same as a restart.
The account changed underneath it
A machine removed in the dashboard, or a plan that no longer covers it, is refused on the next dial with an HTTP status. That is a loud failure rather than a silent one, and the log carries it.