not know what hit one

Verb

To be extremely surprised by an event.

Japanese Meaning

大変驚く、意外な出来事に圧倒される / 予期せぬ出来事により一時的に混乱・ショック状態になる

After the announcement, I did not know what hit one.

発表の後、私は何が起きたのかわからなくて呆然とした。

Sense（1）

To be extremely surprised by an event.

She didn't know what hit her when her boyfriend broke up with her.