Stacking Cable

Hi, I have a stack of Dell N2048 switches (12 in total). The stack seems to be ok and all switches have the correct numbers. My problem is that the 'link' and 'activity' lights on the back of the management switch and the last switch in the stack are off (all others in the stack are fine). The firmware on each switch is the same. I think the cable is faulty - can I replace the cable live or will the switches need to be powered down first? Many thanks