If you are referring to a past incident when someone looked at your diary, it is not impossible to say "It was private", even if it is still private. However, "It's private" would be more usual.
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jhjhpA: Did you see my diary? It was private!!B: Sorry It means now" it's no longer private ?"A common-sense judgement about whether it is still private, based on knowledge of the circumstances, would apply.
vktSo it's up to me, right?As I mentioned earlier, if it is still private (which will normally be the case, I imagine) then "It's private" is the usual thing to say.