Now lets talk again about what exactly went wrong with your site. If your site is still listed in Google but not ranked well, it isn't banned, but it could be penalized or just ranked bad. If it is merely penalized you can file a reinclusion request still (See below) but also understand that it might merely be a temporary thing. Sometimes as Google works on their algorithm sites get dropped for a time, only to come back later. Even a couple months later. I still recommend following the steps above to make sure your site is clean, but sometimes waiting is the best medicine.
Or it could simply be that a major algorithm change has happened and your site is down in the dumps until you can get it ranked better through traditional SEO techniques such has having keyword rich content and good incoming links.
If however your site isn't listed at all you need to file a reinclusion request.
So you've done everything up above, you've waited awhile, and your site is still not listed. Now is the time to file a reinclusion request. It is important to be honest when filing the reinclusion request. If you figured out there was some sort of spam on your website tell them, tell them how it got there in the first place, tell them you've removed it, and tell them you'll never do it again. Also if you just bought the site or domain let them know that you're a new owner.
You may not hear anything back, you might not hear back for a long time. If you don't hear back and things don't improve you can file additional requests, but sometimes a site is just banned and there isn't any going back. If this happens to you, the last thing to try is a complete domain dump. Get a new domain, move all your content & community, and get rid of the old one. Don't do any redirects, don't link to the old domain to the new one in any way whatsoever. Then hope that Google gives you a fresh slate. In this way you lose all your links and external references, but hopefully you also lose whatever stigma is associated with that domain and you are at least able to salvage your content.
This article is Google-centric I know, and MSN and Yahoo, and even Ask.com are viable other search engines, but as I said at the beginning, Google is the 800 pound gorilla and most of these problems occur with Google. However most of the advice here (except the Google Webmaster Tools part obviously) is applicable for all search engines. So if you're having a problem at a different engine you should still follow the advice.
Existing at the whim of a search engine is not fun. These types of problems can happen to anyone, no matter how careful, and so the best advice I can give you is to diversify. Don't have just one site that pays all your bills, make multiple sites, and even multiple types of sites, so that if one fails (and there are more ways for a site to fail than just search engine problems) you have other sites to pick up the slack. When my six figure a year site dropped at the end of 2003, it was horrible, but I was diversified so despite that big hit to my revenue I still ended up increasing overall yearly revenue for 2004. So remember, diversity is the key to reliable long term Internet success.