我不再知道 which means I forgot it. but the structure is pretty odd. We don't say that, but 我不知道 or 我忘记了. In terms of logic, 我不再知道=我忘记了

I understand your struggle pretty much, same things happen while i was learning English . But sometimes , dont focus too much on these little tiny details if you are not preparing any test. Chinese doesn't follow the English grammar rules, so it's hard to find a way to directly translate it. 我知道了: I knew it/ I got it/I see. 昨天我知道，但是我忘了: I remembered it before, but i forgot it now. 我不再知道' I hardly heard Chinese speaker say this phrase...... I hope my answer could be useful for you .

forexample When you call a friend, you can't find the phone number and you can't contact it by other means:我不再知道 Hope this helps anaxth