No, it should have no effect at all if undocked. The only reason that seems plausible, is that the primary junction box (the one labeled "Cap", connected to the reactor) got broken. The reactor controller uses the load value from the reactor itself - but with no connection to the rest of the grid, that value is zero, and the turbine output will be zero as well.
This will also disable battery output (an oversight that's fixed for my other two subs, but not yet for this one), so the whole boat will stay dark until that junction box get fixed.
Can't think of anything else that might have caused it for now, but maybe I have overlooked something.

No, not really. We didn't have any shuttles or weren't docked to a station. It just suddenly happened after we crashed into an island after fighting off some Mudraptors. But I'll check that later. The docking standby does affect the turbine output by any chance? Maybe my friend pushed that button.

@JVne
Were you docked to a station/shuttle by any chance? The reactor will be in a low-power mode unless the "docking standby" button on the bridge is turned off. The auto switch in the reactor window doesn't actually do anything, it will always be under external control.
Please let me know if that helped or not; I just tested it and it seems to work fine.

I love this submarine. But for some reason in my most recent game with friends, the reactor wasn't working. For some reason, the turbine wasn't working on automatic.

I suggest you to bundle different versions together. For example, Anathema mk2 made for entire campaign and Anathema mk1 which is nerfed to be desirable only as a starter sub.
Also, can i recommend looking into Mechtrauma? New mod which can change sub building completely. Maybe you'd be interested in making new subs or versions of existing subs with Mechtrauma in mind.

Okay, I've done some testing and I hear the reactor doom siren all the time. I think a rework of the reactor controller and the surge protection is necessary (until the next update nerfs it all to hell again :DDD). Also, I like your idea about the hauling drone for the Archon, it could help distinguish itself more from this one - and also, I've come to dislike stairs as well, so there will be some interior redecoration as well.
As for campaign progression, I kinda liked the idea of having a small sub like this being able to go through the entire campaign with lots of upgrades, but then again, all three of my subs could fit neatly into the new tier system, with anathema being tier one. Still undecided on how I will handle this though.

Oh, i thought you are gone for good, was checking your online.
I was going to use modified version of Anathema for campaign, but scrapped the idea, cause reworks needed were too much for my sub editor experience.
From testing i can say, that reactors are surely got hit by some patch, less noticeable on Anathema and more on bigger subs. But with Hazardous Reactors all of them meltdown right away.

If i can give you some suggestions, i'd like to have more toys as captain. Anathema is fine, maybe too strong even, inventory and weapon-wise, but next one (Archon iirc?) could have some hauling drone for salvaging wrecks support. And then Avatar could have both battle-capable shuttle and scout/hauling drone. But please, no periscope controlled drones, they are too cheesy. Although, maybe you weren't ever considering these subs as campaign progression set? Anyway, i can dream, heh.

Thanks for the feedback! I haven't done much testing for the new version yet, but hopefully I'll get around to fixing it, as well as reworking the two other subs. That fire thing issue sounds weird though. It can't be extinguished by any means? Maybe it has to do with too many assets being layered on top of each other, I'll look into that.

Could be one of the best, fairly balanced submarines in workshop.
Few issues though.
1. Feels like reactor power may be a little off, maybe new update messed things up? Can you check, please?
2. While testing it and sinking a few times, fire started BEHIND the storage shelf to the right of engine. Can't be fixed ingame, doesn't do anything but kills air condition in engine room.