First of all, there is the Panda penalty. This is when Google decides that your content is of low quality overall. Its quality algorithm is the deciding factor here. For instance, if everything on your site is copy and pasted from elsewhere with little content that was originally developed especially for you, the low quality penalty will likely be on its way. Low quality sites slip further and further back into relevant search pages, making it incredibly hard for your site to show up at all.
The best way to recover from this is the Panda Breakthrough, which reconstitutes information about your site such as keywords so that they appear to be of higher value to search engines and users. It lets your site’s content appear to be optimized and high quality rather than spammy or insignificant. If you have a keyword that has been used so much that you appear desperate for hits, the relevant page will likely be de-indexed altogether during this process.