Question

You said to her, “When was the accident taken place ?”(Voice change)​

Answer 1

It doesn't have one.

Flipping sentences between active and passive is a largely meaningless exercise; as soon as you start looking, you will find examples of both passive form sentences (like the above) that have no meaningful active form and active sentences that have no meaningful passive form ('The sun shines.’ is one of my favourites).

The reason is that active and passive aren't just alternative syntax; they don't convey the same meaning.