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My answer is totally based on my experience and the information you have provided so far. I believe that your website is not under any manual penalty but still due to the low quality of links you have built over the period of time; it is now affecting your rankings. How to get out of it? The best way to start is by auditing your links, get all the links from Open Site Explorer, Google Webmaster Tool and if you can invest then get the link profile of your website from Majestic SEO as they have the largest index of links among all. Audit the link profit and separate the bad links (you can use software like Link Detox and Link Risk here) from the profile and try to remove as many bad links from the link profile as possible and create a disavow file for the rest. Upload the Disavow file in Google Webmaster tool and you are half done! This way you have told Google that you don’t want a link juice from the list of links that you have updated in the disavow file. The step two is to create regular and kick ass content on the website and get quality links back to the website. After submitting the disavow file your core job is to create quality content and get high quality links naturally that points back to the website. Hope this helps!

Closing thread, OP reported elsewhere this is fixed.