This what happened recently to me:
My Google AdWords campaign suddenly stopped working after several month. No more impressions, although I did not change anything.
After contacting the (very good) Google Support, they told me the following (in German, translation follows):
Ihren Angaben zufolge können Ihre Anzeigen nicht angezeigt werden. Dies liegt nach unseren Erkenntnissen daran, dass ein unberechtigter Zugriff auf Ihr Konto stattgefunden hat. Als Vorsichtsmaßnahme wurde Ihr Konto daher zeitweilig deaktiviert. Es findet eine Untersuchung statt, die mehrere Werktage in Anspruch nehmen kann. Nach Abschluss der Nachforschungen aktivieren wir Ihr Konto wieder und erstatten Ihnen alle Kosten, die aufgrund des unberechtigten Zugriffs entstanden sind. Das Ergebnis unserer Untersuchung wird Ihnen anschließend per E-Mail mitgeteilt.
Roughly translated to English this reads:
According to your e-mail, your ads can not be displayed. This is according to our findings that a unauthorized access to your account has taken place. As a precaution, therefore, your account has been temporarily disabled. There is an investigation, which several working days can take. Upon completion of the investigations we activate your account and you reimburse all costs incurred because of unauthorized access arose. The result of our investigation will be followed by e-mail.
The interesting part is that I, as the administrator of the Google AdWords account did not see any indicator to this disabling. I.e. I could work with the account just as I did normally, only that my ads (which where shown as „active“, too) had no impressions.
After a few days and some e-mails between the Google AdWords support team, they activated my account again. Until now I have no clue what was their indication of the „unauthorized access“, since in my eyes, such an access never happened.
Bank Account Error
One more side note: When I tried to modify my payment settings (bank account or credit card), an error message always told me:
Probably this was due to the disabling of the account. A week after the account was unlocked, I succeeded in changing the bank account data (changing of credit card was not test, but I assume it works, too).