Your game will auto-resign if it's your turn to play and 3 days elapse without you doing so. Likewise, your opponent will auto-resign if it's his/her turn to play and 3 days elapse without him/her doing so. We imagine that this is probably what happened. If you don't think this applies to your situation, please contact us.
(Note: the 3-day period is subject to change, but at present that is the time allotted for playing a move when it's your turn.)
The Panda Helpers feature attempts to avoid these auto-resignations by playing moves on behalf of players nearing auto-resignation. However, a match where neither player is playing will be auto-resigned. You can learn more about Panda Helpers here: https://spryfox.zendesk.com/entries/21758069-What-are-the-Panda-Helpers-and-how-do-they-work-