Today morning you were seen on the first page and evening you are not found on the SERP When Google does not return with your site when searched for using the site (site: your site url) command There is a sudden decrease in your page rank.
Making text or links hidden by coloring them in white When two different pages and content are made one for search engine and other for user this is called Cloaking in technical terms. Use of 3-5% keywords are considered as legal. More keywords are considered as keyword stuffing and are disliked by Google. Having duplicate content by creating domains, sub domains or multiple pages Use of pages with mischievous behavior like installing mal-ware Doorway pages with duplicate content