What's changing?
Beginning March 26, 2018, once an AdWords account reaches 15 months without any spend, it will be canceled in order to speed up the AdWords experience and help users stay within the manager account limits. This also means that right now, if your account has reached 15 months without spend, it meets the criteria and will be canceled. After an account is canceled:

• get requests for ManagedCustomerService will no longer return the account.
• Any AdWords API request that specifies the account's customer ID in the clientCustomerId header will fail with an OperationAccessDenied error and a reason of CUSTOMER_NOT_ACTIVE starting with v201802, and a reason of ACTION_NOT_PERMITTED for earlier versions of the AdWords API.
• Any unspent prepaid amount will be refunded back to the advertiser.
• All ads in your account will stop automatically within 24 hours.

Note: Test accounts will not be canceled.

What should you do?
If your application issues AdWords API requests against inactive accounts, make sure you download any relevant data for those accounts before the cancellation process starts. For example, you may want to run reports to gather historical stats for the date range where the account had activity.

If you are using an inactive account in order to test your integration with the AdWords API, consider creating a test account instead. Test accounts will not be canceled by the ongoing cancellation process.

To avoid issues once the cancellation process starts, you can proactively cancel accounts as follows:

• Log into your manager account.
• Go to the Accounts page, where you can adjust the date range and review the performance metrics for each account under the manager account to help you identify inactive accounts.
• Check the box next to each account you want to cancel.
• Select Edit -> Cancel account.

To reactivate an account, follow the instructions in the AdWords Help Center. If you are logged in as a manager account, you can show canceled accounts by adjusting the filter in the upper left corner as follows:
Note: If the account is under one or more manager accounts, you will only be able to reactivate the account if doing so will not exceed the manager account limits.

If you have any questions, please contact us via the forum.
- Josh Radcliff, AdWords API Team

Source: Google Ads Developer Blog