vacantly; blankly; absentmindedly
openmouthed; with one's mouth wide-open; gaping; flabbergasted
with a whack; with a thump
急な質問にぽかんとしてしまった。
I was left blank by the sudden question.
彼はぽかんと口を開けて空を見た。
He stared at the sky with his mouth agape.
説明を聞いてもぽかんとしていた。
Even after hearing the explanation, he looked vacant.