I closed my book (by mistake) and don't remember where I stopped reading.
I closed my book (by mistake) and don't remember where I stopped reading.
もし本を故意に閉じてどこまで読んだかわからなくなったなら
I closed my book... となり
本を誤って閉じてしまってどこまで読んだかわからなくなったなら
I closed my book by mistake ... と言えます。
don't remember where I stopped reading で「どこまで読んだか覚えていない」
ご参考になれば幸いです!