Have you pushed your link building too much and received a swift kick in the rankings?
Over the years I've had plenty of websites and I have had a few get a nice little penalty from Google because I went a little too hard in the link building department lol. Now some of these websites I got the penalty for randomly and others I did on purpose. Now you're thinking "Why the Hell would you get a penalty on purpose?" well it's because I wanted to do a lot of testing in order to see how hard I could push my link building and also to see what types of links get the penalty quicker vs links that will rarely bring in a penalty Always be testing!
The penalties can be anywhere from a slap on the wrist to a full blown blacklist. I've gotten blacklisted only once, and it was pretty difficult to do. So if you get your website blacklisted then you've been doing something very naughty and you're probably the person who talks bad about SEO when it happens lol. But if you're the person who got a slap on the wrist, it's likely you didn't realize you were doing something you shouldn't be *cough* link wheels *cough*
Getting black listed is pretty much a death sentence for your website. You should pretty much buy a new domain, hosting and set up a new website because you'll never be found in the search engine that hates you. If you have a penalty you can usually come back from this downfall, but it takes some time and effort. If you don't know how to figure out what went wrong, I would highly suggest contacting a good SEO company, or look for someone here on SeoClerks who helps with penalties, and see if they know what went wrong. Write down a list of things that you did prior to getting the penalty and that will streamline the process and quicken everything for the person who is doing the research to figure out exactly what happened. Even if you give them loads of info, they might not be able to figure it out since they don't work for Google, Yahoo! and/or Bing. But usually just getting a list of what you did will give the person a good idea of what went wrong and what needs to be fixed