The same thing happened to me last month. I've always admired Google for making low-cost, self-serve options available to the small customer. Whatever's going on with the AdWord account suspensions, it feels very contrary to that admirable image I've had of the company.
My account was opened about 8 years ago too and had low levels of usage. For the past three years, there was no activity. I actually tried to run a campaign last year but found that it would never leave "Eligible" status. It was only an experiment, and I had higher priorities, so I didn't pursue it. There is no balance due. The two sites on the account are tech consulting (since 1996) and consumer electronics reviews (since 2010), and there's never been anything objectionable on either account. (Only I've owned the domains.)
In November, I got a notice that my on-file credit card had expired. I updated it, and the next day I got an account suspension notice. When I called, the rep was very nice and led me to believe that there must have been a mistake, which was what I expected too. I even asked him to look at the site, which he did and said it looked fine.
However, after a few days I was told that there was no mistake. Furthermore, the rep said she could not tell me why, just an unspecified violation of terms of service. I asked what I should do to fix the "problem" and was told that there was nothing that I could do -- the suspension was unconditional and permanent (?!?). I was told not to open another accounts as it'd be suspended too.
They made me feel like a low-life scoundrel. I'd done nothing wrong, and there was nothing to do about it. Ever.
After a few days, the following notice appeared on my AdWords account:
"Your Google AdWords account has been permanently suspended because your billing information in this or a related account could not be verified."
The bit about "or a related account" rang a bell: In my AdSense account, I'd never been able to verify my corporate checking account via their ACH deposit method. Despite it being over a decade old, with a major national bank, and having used it for ACH transactions many times in the past, I couldn't get the AdSense verification process to work. I gave up and accepted the check method instead.
My best guess is that the failure to verify the AdSense account for ACH became a risk signal of some sort on the AdWords side.
I went ahead and opened a brand-new checking account at a new bank. I searched around and found lots of people encountering issues with AdSense account verification. I took extra care and used all of the suggestions and got the new bank account to verify for AdSense ACH.
I've not yet re-contacted AdWords. They gave me no hope first two times around. I've been partly waiting to see if the AdSense account change might propagate through their system, but really, the experience has left me feeling rather defeated. I've mostly been hoping to read somewhere of what to do, some sign that there had been mistakes in their assessments that are now corrected.
I'll update this thread if I learn anything more. Viggen, or anyone else, please do the same if you figure anything out. Thanks.