Banned by Google

Actually, you probably aren't banned.
I did start writing a long piece here about how *this* could be happening and *that* could be happening but to cut a long story short, realistically you probably aren't being banned by Google at all.
Site dropped like a stone in the rankings? Happens all the time.
Site unaccountably vanished? Give it a week, maybe a month, or two, most likely it'll pop up again pretty much where it was. Both of the above scenarios occur on a regular basis in Google and have done for years. It's got a lot worse since BigDaddy but it has always been there.

Yes but in my experience only rarely. If you read the SEO forums you'll see repeated complaints by people, me included, of spam-riddled competitor sites still maintaining their high presence in the rankings despite copious and frequent use of the Google abuse form by their accusers.
The only time I've known it do any good was when I complained about a site that was no more than a redirect to a very hard porn site. Hiding as it was behind an innocuous title it was clearly underhand and Google deleted it from the index a few days after I complained and emailed to thank me for drawing it to their attention.
I wish it was always that easy.

However, at least one of the sites I was referring to above finally did get banned from Google. Gone. Kaputski. De nada. Nailed to the perch. Banned. And it absolutely deserved it too, blatant CSS spamming, z-index spamming, a veritable compendium of pointless spam.
Guess what, though? It was back in a couple of weeks, just like BMW was! How? They got rid of all the spam, filed a re-inclusion request with Google, promised to be good, and hey presto! restored to their former high-ranking position in the SERPS.

I know there are a number of doom merchants in SEO who will gloomily forecast that you'll be banned for every minor infraction of Google' Guidelines. Comment spam? You're gone. Noscript spam? He's history. Keyword stuffing? Don't forget to write.
Hey, people, these are guidelines, not absolutes! If the site has good content, it doesn't mean it'll automatically get banned because your webmaster (or whomever) has made some ham fisted attempt at spamming! It's certainly not to be encouraged but there are those in this industry who will demonise every petty infringement of what are after all only guidelines in order to make themselves appear to be authorities. My advice? Don't believe them.

If you still feel you have cause for concern about being banned by Google, you can try my SEO Services and I can run a diagnostic. Or you could call me direct on (0)20-8224-5976.

But no, in the end, you probably aren't banned. Unless you have a site filled with nothing but spam and no real content, or you have a site blatantly masquerading as something it isn't, or you are a disguised redirect to a porn site, then in all likelihood you haven't actually been banned by Google.
Keep your nose clean, and you probably never will be, and even if you are unlucky enough to be banned for some minor infringement, clean up your site, file a re-inclusion request and if your site is otherwise righteous, I can see no reason why you shouldn't get (eventually) back in.