SEO campaigns fail because of too high expectations, too little budget, and decisions which are made based on assumptions that were misunderstood. What do you then if SEO goes wrong?
First is to find out what is the biggest problem. You may look at Google Analytics and see when the drop-off occurred. Google console (formerly webmaster tools) would also be another way to see if there are any notifications you failed to notice. These are usually good indicators as to where you may have gone wrong.
Read more here: https://www.seolosangelescity.com/what-to-do-if-seo-goes-wrong/