I did this a few months ago and got this email...
We recently contacted you regarding a past due balance for your AdWords account 000000. The outstanding balance is the amount of USD 410.96 for this invoice with an overall outstanding balance of USD 631.89 on the account.
If payment in full is not received within fifteen days, we will forward your account to our external collections agency.
To reactivate your account, please enter your payment details. We ask that you re-enter your payment details even if they have not changed; entering this information causes our system to attempt to charge you again using the payment details you provided.
To enter your payment details, please log in to your account at hxxps://adwords*google*com/select/. Select the 'My Account' tab, and then click on the 'Billing Preferences' link.
If you prefer to settle the full outstanding balance on your account with a single payment, you may also make a one-time payment by cheque or Electronic Funds Transfer. On such payments, you'll need to include your AdWords order number, 00000000, as remittance advice. If you do not reference your order number, your payment may not be properly credited to your AdWords account.