Are you sure the autopilot computer is powered? After verifying correct polarity have you moved the fuse to the correct position?
Are you sure the SimNet wires are connected to the correct terminals?
Is the SimNet configuration correct with terminators at the right positions?
Also double check the SimNet connections at the autopilot computer terminal block, maybe even cut the loose wire end, strip the outside insulation back and redo it. Could be that one of the small wires got nicked and is preventing communications or causing an intermittent fault.
Did you connect the red and black wires in the SimNet cable to the corresponding terminals on the AC42? These are the "Net-C" and Net-S" wires for powering the SimNet backbone with 12V. You should not have two 12V power sources on the SimNet. Try leaving your other SimNet power source connected and disconnect the red and black SimNet cable wires at the autopilot computer terminal strip.