in which stste of page life cycle view state is loaded

View State is Save only when the successfull submition of the page. During Load View State Event, the view state data that had been saved from the previous page visit is loaded and recursively populated into the control hierarchy of the Page. It is during this stage that the view state is validated.

View state can be invalid for many reasons --> Ref: http://msdn.microsoft.com/en-us/library/ms972976.aspx