What would cause the audio signal from not working? while the the monitor was still powered up, but with the master volume control turned all the way down, I un-pluged a cable from the signal source which was an effects unit. There was a popping type sound that occured, after that the audio signal would not work. what has happened, and what needs to be replaced to make the SRM150 audio signal work again?

Audio Players & Recorders - Mackie - SRM150 Compact Active PA Spkr ea