1990 Ford Bronco speakers stopped working

I was listening to my radio and all of a sudden all of my speakers went out. i turned off the radio and turned it back on and the speakers made a "pop" noise but they still didn't work. This is the second time this has happened. the first time it happened the speakers ended up coming back on. and now they are off again and didn't turn back on yet. could anyone tell me whats wrong or how to fix it?

Thanks, Chris