I agree with Gael's suggestion to check that the ME state is set to always ON in the ME configuration. Apart from that, before you unplug the network cable, were you able to ping AMT regardless of the host power state?
Remember that once you unplug the network cable and plug it back in, AMT would require some time to recover from the event and once the network stack is back in operational state, you will be able to ping again. Are you using static IP or DHCP configuration?
One other test I would do is after you plug the cable back in, give it some time and try the ping. If the ping does not work, try accessing the web UI to test the connectivity. Hope this helps.