None of the top level dnsX.nic.io servers seems to be responding to requests right now. More and more users are seeing the lights go out on .IO domain hosted websites.
For Planio (<a href="https://plan.io/" rel="nofollow">https://plan.io/</a>), all accounts are available via yourname.planio.net as an alternative while this outage lasts.
Seems it's not the first time this happens: <a href="https://news.ycombinator.com/item?id=5838670" rel="nofollow">https://news.ycombinator.com/item?id=5838670</a>