Google announced that it will begin automatically canceling AdWord accounts that have been inactive for 15 months or more. Google will send an email to all account owners whose accounts are being canceled.
The reason behind Google’s automatic account cancellation for dormancy appears to be speed. This statement comes from Google’s Twitter account announcing the pending cancelations:
Starting next week, we’re helping speed up your AdWords experience by automatically deactivating accounts that don’t have spend in the last 15 months.
Google will allow you to reactivate a canceled account at any time. Note, however, that Google will cancel any account that falls dormant for 3 months. Your best bet to avoid cancelation and losing access to your account via Google AdWords API is to publish an ad, sooner rather than later.