Do the subs still work or are they popping too? If it's only the fronts/rears or only the subs popping look below. If all of the speakers are popping then the pre-outs on the head unit may have died, again look below but try without disconnecting all of the speakers first.
Disconnect all speakers in the car at the amp, grab a spare (speaker) from inside or from a mate and connect that up. Hook an ipod/discman up to the amp instead of the cars head unit and see if you get sound. Make sure to turn the ipod right down before connecting it up of course.