The CPU7 is not a PLC. It is the motion control board. You probably have either a SERVO3IO or a DC3IO unit, combining the PLC hardware with the servo amplifier.
The most likely causes of a 401 fault are loss of logic power to your PLC unit, or a problem with the fiber optic communication between the CPU board and the PLC.
Find out what type of PLC unit you have, and see if there are any indicator LEDs on it.