Suppose your had a pristine ranking one day, and the next you can't find it in the search engines? Your traffic dries up because nobody can find your site any longer. Your thousands per month in income becomes zero, and the debtors start looming on the horizon. What are you going to do?

You need to investigate just why this happened. It might be simple, or you may have strayed outside the bounds of the content guidelines published by the search engines.

The first thing to do is determine whether your site is totally missing, or just has a low rank.

In Google, if your search term is ":site mydomain.com" then Google shows you every page it has indexed for your site. If it shows zero pages, when the other day it showed thousands, you're being blocked. If the ":site" search does show pages then you're still in the search engine.

For example, when this happened to me the Google toolbar still showed a pagerank for my site, and the ":site" search showed my pages were still indexed. But my traffic had dried up, and the site wasn't listed at its usual second-place rank for the primary search term. I eventually determined that a few days before the site plummeted I'd broken the site so that none of the pages responded for several days. Google (googlebot, that is) must have swept through the site during those few days and seen all the pages were missing, and dropped them out of the index. What corrected the problem was to submit a sitemap, Google quickly reindexed the site, and it quickly returned to the second-place in the results it had always occupied, and visitors began to return.

Maybe you've been playing with "search engine optimization" tricks, or you've hired a SEO specialist. There's various tricks that will be done which aren't kosher by the guidelines under which the search engines index sites.

See, the search engines establish some rules governing the ranking of search engine results. Those rules are in the ranking algorithms used in the search engine. The search engines do this to attempt to return relavent results to the users. However there's a lot of people trying to "game" the search engine game, trying to gain high ranking in the search engines for websites that are basically crap. There's real money to be had, because someone putting up a crap website that ranks highly because they've correctly gamed the system, can earn revenue through putting advertising in front of the people spoofed into entering their site.

The search engines constantly adjust the algorithms to try and spot such sites and penalize them out of the results index. It's a constant game between the SEO tricks and the search engines.

Filing a reinclusion request: Posting by Matt Cutts describing what to do if Google has penalized you as a spammer.

Anatomy of a Successful Reinclusion Request: SearchEngineWatch posting about how to work the reinclusion request process.

http://www.google.com/webmasters/guidelines.html - Googles guidelines to webmasters.

Coping with Search Engine Penalties: Article about the type of search engine penalties

Checking If You're Banned On Google: Overview of how to determine your status