If your C70 is powered from the Seatalk network, then it's likely that the cable you found unplugged from your autopilot had broken the circuit and hence no power to your chart plotter. As for the lack of input from the chart card, it's likely that the contacts on the card or more likely on the card reader are dirty or corroded. You might manage to clean those contacts sufficiently to get it working by repeatedly inserting and removing the chart card, or with very gently wiping the card reader contacts with some very fine wet and dry sand paper supported on the end of some stiff card.