SEO issues

Hello,

I am using Yoast, which I have heard some really good reviews about, and I am sure its a great plugin.

I followed some books examples and applied their concepts on the SEO using Yoast.
My page was listed within the first page when I performed the suggested changes.

After a week, without doing ANY change,, my site has fallen on the 3rd page on all searched I used to be first page.
Note that the website is brand focused, so it doesnt make sense why I have fallen so quickly.

Would it be possible to assist me of the reasons why this has happened, and how Yoast can help me out?

https://wordpress.org/plugins/wordpress-seo/