Seems like HE forgot to renew their TunnelBroker.net domain :(
To keep DDNS working you can add the following to your /etc/hosts:
64.62.200.2 tunnelbroker.net ipv4.tunnelbroker.net www.tunnelbroker.net
Retrieved via dig @ns1.he.net tunnelbroker.net
P.S. HE support is already aware so don't bother them with emails