Google AdWords on Twitter has mentioned that they will be automatically deactivating accounts that have no spend in the past 15 months. The good news is that you can reactivate your Google AdWords account at any time.
When your account is deactivated, you will receive an email notification that your Google AdWords account has been cancelled.
If your AdWords account has been inactive for more than 15 months (meaning your account hasn’t spent in more than 15 months) your account will be automatically canceled. Note that there are other reasons an account can be automatically canceled.
You can read more about cancellation of inactive accounts on Google AdWords Help.
You can reactivate your Google AdWords account by simply signing in and reactivating it. However, if you continue to have no spend in the next 3 months, then your Google AdWords account will be deactivated again. If you cancelled your Google AdWords account on accident, you have 2-3 weeks to recover it.
You can find out more information on reactivating a cancelled AdWords account on the Google AdWords Help section.