For some reason the mic wasn't working just now.

Literal

Just-now why-somehow mic [subject-が] not-turn-on.

なぜか 'for some reason' signals unexplained failure. マイク 'mic/microphone' is a clipped loanword. 入らなかった 'didn't turn on / didn't activate' — 入る for electronics means 'to turn on, to activate.' Simple, natural troubleshooting complaint.