I would open a support ticket with NGS to ensure the failed node is ok. You don't want to try doing a giveback and cause other problems without diagnosing the failed node. If it did a failover there is probably a good reason why it did so would strongly recommend having NGS take a look at the logs to be sure you don't have any failed loops, h/w, etc.
Once the secondary controller has taken over the failed controller you can only telnet/ssh to the secondary. You can however execute commands on the "taken over" controller by running partner from the console.
Have you done DataONTAP upgrade lattely? In latest version of DoT you should increase takeover detection time from 10 seconds to 15 seconds. Please take a look at opitons cf.takeover.detection.seconds.
Unfortunately you cannot telnet into the node that has been taken over. Its essentially in either "waiting for giveback" mode or its completely down. If you have access to MYautosupport you should be able to take a look at all the logs avaialble for both nodes. Again , I would strongly recommend getting NGS involved if you haven't done so already. Good luck!