An adverb meaning 'for some reason' or 'somehow' when the reason is not clear or unknown.
なぜか彼のメールに返信が来ない。
For some reason, I haven't received a reply to his email.
今日はなぜか気分がいい。
For some reason, I feel good today.
なぜかその店は閉まっていた。
For some reason, that shop was closed.