How is this past tense??
The infix ja indicates the immediate past in the negative tense, i.e. corresponding to me in a positive sentence:
Kitabu kimepotea: the book is lost.
Kitabu hakijapotea: the book is not lost.
Hope that helps.
The ja implies that the book will be lost at some point.
The book is not lost (yet)
So this should be: "the book WAS not lost!"
That would be 'Kitabu hakikupotea'