Pioneer car stereo (FH-P515MD) has lights, seems to run, speakers are fine but no sound comes out

Help! I have a Pioneer FH-P515MD head unit. It was playing a CD fine until I heard a pop sound then suddenly the music was gone but the lights on the display were still going and the CD seemed to still be playing. I thought the speakers had blown but I took them out and tested them on another stereo and they work. Any suggestions so I don't have to go buy a new one (which may not work either depending on the problem)? Cheers

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