iPods are really really thin and they can be easily broken with a hard hit. A simple contact could have come loose or maybe a bad connection happened and broke critical parts on the logic board. Unfortunately as you have already tried replacing cameras, you've probably re-install the new parts correctly which means that it might be an issue without a possible repair. Saw this a couple of time when the iPod Touch received a pretty hard it. Some iPods I replaced screen had non-working cameras after, as I care a lot about the way I repair the device, I know for sure that the damage happened as the screen shattered.