Er, let me think. Sorry, my mind's gone blank.
让我想一想。抱歉，我大脑一片空白。
I've filled in this form incorrectly. Can I have another blank?
我把表格填错了. 再给我一张空白的, 可以吗?
My mind/memory was a (complete) blank I couldn't think of a single answer.
我头脑[记忆]里（完全）是一片空白--连一个答案也想不出.
