Over the years search engines have changed. These changes will continue as long as we have a free and open Internet. People seem to favor and use the search engine that will return the best results based on content.
Many years ago it was popular to use SEO methods that today will get your website banned from search engines all together. These methods include link farming, overloading keywords, gateway pages, and many more. Search engines always catch on to these black hat methods, adjust their database accordingly (Panda), and then add the method as a violation to their terms of service.
If SEO work was done to your site in the past, then it needs to be done again. Old methods may now be harmful in your website rankings. Also newer methods not used or known about in the past will now be beneficial to your website.
Staying on top is a never ending project, but the results pay off. The internet business world changes at a much faster pace than retail store location businesses. It was not that long ago that fast food places did not accept credit cards. It was not that long ago that cell phones did not have data plans. I could go on and on to find an advancement that happened recently that now seems impossible to live without. Put time into your website on a regular basis and research the best way to use this time.
If your website was banned in the past, or currently banned for any black hat methods, then you must make the appropriate fixes and contact the appropriate search engine(s).