Why some pages dissapeared from Google Index ?

Hello everyone,
So, for last 7 days my website was getting a good number of visitors everyday. Today, I have noticed that my traffic has greatly decreased.
Later, I found why it greatly decreased. The problem is that some pages has been dissapeared from google index. But why ? Is it temporarily problem or … ?
P.S. My website is unique enough with unique content etc.

See original post by outanmax