Why Many Pages of My Site Gone from Index Google..?

Hi all..My name's Hery..Fisrt of all..I'm so sorry my english is terrible..;)  But I hope you can understand my words..

I have a big broplem and I hove somebodey wioull help me ..Since yesterday my site http://www.plazabajumuslim.com/ dropped in google search result..Not too much but it affect the traffic..Then I try to search in google site:my site url then I surprice since many pages of my site are gone including the blog post wich have 300-600 original words articles. SO..

But when i check the blog post url in google..For example : http://www.plazabajumuslim.com/apa-itu-hari-raya-idul-fitrilebaran-dan-bagaimana-kaum-muslim-merayakannya/  They are still indexed..They even  get relatively good rank in serp. So I really wonder.. Why this happen...? Please somebody tell me why this happen and what I have to do to fix this...? I appreciate any help or advice...

Thank You Very Much..

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