blown fuse

Check the radio fuse under the steering wheel fuse box and replace it if you see that it's blown.

bad amplificator

A bad amplificator can fail to deliver power to speakers. 

To test it out, remove it and replace it with another testing one, and see if there is sound then.

If yes, throw the old one away and install a new one.

bad wires

A loose or bad wire connection can fail to deliver power, thus no sound.

Check also for shorting wires.