Why can't I record while listening to the background music?

There’s one track for background music and another for the recording.
I have got no problem with recording before.
But today when I open the audacity and try to record while listening to the bg music, I can’t.
please reply me ASSP!!!
Please and thanyou… urgent…

OK. Extrapolating. You’re trying to overdub and your microphone vanished? Audacity will “lose” a USB microphone is it becomes disconnected, even briefly. Restart Audacity with the microphone connected, or Transport > Rescan…


okay, thank you. I fix it just now (: