Google Banned Checker
Sometimes, a web site can be Banned (blacklisted) by Google. Usually, this happen when you implemented unhallowed Search Engines Optimization tricks such as Cloaking, content hiding, fancy re-directions, "shadow" domains etc ...
Google is quite precise about it : Search Engine Optimizers & Google.
This tool will tell you if you where unlucky enough to be blacklisted by Google.
If this is the case, it will probably take some time to get back to normal.
You should first make sure nothing unhallowed is being used any more on your site.
If you think you did not implemented ...