I am more familiar with the Raymarine units than the Simrad units, but, if it is a relatively modern unit, I think the basics are the same.
There should be a "SimNet" bus, which is like a Raymarine SeaTalk-NG bus. It is basically a powered NMEA-2000 data bus. Although it is a data bus, it also supplies the power to instruments that don't demand a huge amount. For example, it'll power things like Autopilot control heads and wind sensors, but not big things like pumps or autopilot computers.
The way the bus works in the most general sense is that there is a relatively thin data cable that has some ends on it that, when plugged in, make a waterproof seal. This cable can be connected to hubs to allow it to interface devices. Some of these hubs have 3 connectors and are called Ts. Some of these hubs have a bunch of connectors and they are called hubs or multijoiners or whatever.
The upshot is that when you look at it you want to imagine a SimNet backbone with devices attached to it by spur cables. Normally, there shouldn't be open connectors -- these should be "terminated" with "terminators", basically a type of cap.
As I mentioned above, the SimNet bus is powered and supplies power to devices -- such as autopilot control heads (but not computers or pumps). So, if your control head is not turning on, it is probably because the SimNet bus, or the part that it is on, isn't getting power.
There is usually a power cable connecting to the SimNet bus. It basically plugs in like a data cable (but it is not a data cable) and powers the devices on the bus. Because it almost always shares a breaker with something else, it almost certainly has a fuse attached to it. Again, I am less familiar with SimNet, but my memory is that these power cables either have red rings around the plug side, or are attached to an integrated T-connector with red rings. The other side of the cable will be the two wires that get connected to power, hopefully via a fuse.
If your network was set up primarily for Autopilot, it probably has a simple structure. There may not even be a separate bus. In cases like this, they normally use one multijoiner (hub) and connect it to power as well as all of the autopilot related devices, e.g. the computer, control heads, rudder transducer, and the wind sensor if you have one, maybe the mutli-function unit/plotter.
But, at a certain point, once you start getting into multiple helms, more devices, etc, one hub isn't enough, and they get connected by cables, etc. Then there is more checking around to do.
Regardless, the most likely problems probably are:
1) The control unit got disconnected from the SimNet bus
2) The SimNet bus's power cable got disconnected from power or the fuse is missing or blown
3) The SimNet bus is disconnected somewhere between power and the control unit.
If you've got other small insturments connected to SimNet, such as a wind speed gauge, that might give you a hint as to whether or not some part of the bus is getting power.
You might also be able to pull apart the connections and test for power along the way, such as right at the control head. This looks like the pin out:
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https://seatronic.no/media/catalog/...3525d08d6e5fb8d27136e95/s/i/simnet_tig_03.jpg
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With the "key down" it looks like red is to the left and black is above red.
Hope this helps!