How to get my keyword Top rank back after dropping away

You can check out your spam backlinks of your website and work on building good and quality backlinks to get back your rankings in search engines. I think you need to wait till another Google update to get your keywords back to the top position again.
 
Best suggestion is to visit Web Masters tool and check if there is any message from Google. If it says Google sees your website as suspicious and black marked, then this could be serious if not, the most likely causes are Google Update effect, or your website might be possibly hacked (this is more so if you are using wordpress).

Either way there is no need to panic, If your website has too many Paid links then you can remove them and ask for reconsideration to Google (Via Google Web Masters tool), if it is hacked, it is even simpler, ask your developer to patch it, or upload an older version (from backup) and your website will slowly recover. All the best...!
 
carrying out all the off page techniques like directory submission, social bookmarking, article and blog creation and their submission, etc. will help in increasing the backlinks, but at the same time if the keywords are well mapped in On page and while doing off page submissions you will get hype in a short time in the search engine (this should be done ethically with the best relevancy of the keywords) and then as the site gains popularity by gaining the number of visitors it helps in increasing the page rank. you can also hire experts from SEOclerks who can help you in increasing page rank in cheap rates.
 
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