Too often in webmaster forums and SEO JunkieAnonymous meetings around the world do you run into the situation where a webmaster panics after having their rankings in Google disappear. And why shouldn't they panic? Google is the 800 pound gorilla, a good ranking in Google can make or break your site. If your Internet ventures are your only source of income you could be screwed. I speak from personal experience, I've had a 6 figure a year site drop from Google suddenly, it isn't fun.
If you find yourself in this situation stop, take a deep breath, and try to figure out what could be causing this.
The first thing you should do is check your site in Google Webmaster Tools, you are setup there aren't you? If not, its easy, and you should take this opportunity to setup all your sites. Don't be afraid and think you actually have to make an RSS or other format sitemap for each of your sites, you don't (though you should). To get the basic reports you merely need to authenticate that you're the site owner through any one of a handful of methods.
Inside Google Webmaster Tools you will be able to easily ascertain if your site is in Google's index but merely ranked very low, or if it is not in the index at all, and the difference here is important (more on that later). Google may also report a ban or penalty here if your sites are setup in their system.
There are also tools for double checking important things like your robots.txt file (you didn't accidentially block Googlebot did you?), as well as 404 errors or anything else that could be hindering indexing.
Eventually if you figure out that your site is penalized or banned, it is through Google Webmaster Tools that you will want to submit a reinclusion request, but more on that later.
You should also at this time make the full RSS sitemap for your site, it might help it get reindexed and it is certainly worth it if it does.
If it turns out that your site is out of Google entirely, but no crawl errors or robots.txt problems are found, you could be banned or penalized. You can start being proactive now and assessing possible reasons but also understand that you could merely be experiencing an indexing or updating hiccup and in a few days things may revert to normal. So don't too crazy in implementing the following advice quite yet.
The first thing you should do is look at the source code for your pages, especially if you are not the designer or if you hired a third party SEO company. I know you like to think that those you hire know what they're doing, but in this industry they are often clueless when it comes to modern SEO methods. For instance your designer may have good heartedly added hidden text to your site thinking he was helping you, not knowing that hidden text has been a definitely no-no since the late 90's and anyone doing such a thing now risks being banned.
Additionally third party SEO companies are often built more around selling their services first, and delivering a quality service a distant second. So they aren't always up to speed on modern methods and guidelines either. What is worse sometimes they purposely hide code in your site to benefit their other clients. One infamous SEO company who did this ended up having all their clients banned.
If you are not technically proficient enough to analyze your code for these mistakes I recommend visiting a webmaster forum and asking the experts there for a quick check.
Of course if you yourself added some things like hidden text, deceptive redirects, cloaking, paid links, automated link exchanges, or anything else against Google's quality guidelines, you should remove it.