Ranking on Google Dropped suddenly???

[SEO]

Hi All,I am an webmaster of highly ranked website : global-opportunities (dot)net My top keywords where my site ranking high:study in UKHigher education in Ukbest universities in uk etc.. I have updated new pages on my website into the university section around 9 new universities uploaded so far and total new pages goes to 45. I am also running a PPC campaign for my website. Now i hv checked that ranking on my top keywords is going down , Please let me know the reason. Also i have found the 4xx errors on the webmaster tool and also fix the unreachable urls from the servers. Where is the problems please let me know in brief.Thanks & Regards