When preparing for a hike, I ended up putting my whole backpack in the fridge to cool the water in the Camelbak inside of it. The hike was canceled, and I'm the kind of guy that doesn't really care that there's a backpack in my fridge for a month. Only now have I realized that I had my Sigma 35mm f/1.4 in it. I have removed it from the fridge. My backpack is no worse for wear, but I worry the lens might be damaged. Should I be concerned? The fridge is 43° F (6° C).
Storing the lens in the refrigerator for a month or even indefinitely at 43° will not harm it in any way. What could potentially damage it is removing it from that environment without taking adequate precautions.
Any time you move your camera or lens from a cold environment, such as your refrigerator, to a warmer one you should be sure to place it in a container of some kind so that the temperature change is gradual enough that moisture in the warmer air does not condense on or, more importantly, inside the lens. This can be as simple as a zip lock bag or a camera case or the backpack it was stored in when it was in the refrigerator. If you do have moisture condense inside the lens it will usually leave water spots behind. Moisture can also promote the formation of fungus inside the lens. If the moisture is extensive enough, it can even cause havoc with the electronics of the lens.
This question and the answers submitted cover how to prevent condensation when going from cold air to moist hot air: What precautions should I take when taking a camera into humid conditions?
After the fact, you need to dry it out as best you can. See How do I dry water condensation off my camera?
I wouldn't be too worried. Many cameras are rated for use in temperatures as low as 0 degrees Celsius and some pro bodies even lower. Lenses tend to be specified in the same temperature range. As for this particular lens I couldn't find the storage and operation temperatures unfortunately. However I own many similar Sigma lenses and live in Sweden where I use them during the winters and never had any problems with the cold weather (it can be a lot colder here than in your fridge).
Your lens stayed in the fridge for a month but as the temperature there isn't below the freezing point of water (which could otherwise be a problem if condensed water freezes inside the lens) I don't think there's any danger.
Did you remove the lens from the fridge by placing it in a sealed bag? This prevents water to condense on the very cold lens elements while the lens is heating up too room temperature. If you didn't there is a change that water ended up on the lens surfaces within the lens and this can promote fungi growth. Inspect the lens surfaces and if there are spots inside the lens that don't go away consider taking it to a service center for cleaning.
I would continue to use the lens as usual and if there has been any damage time will tell. Of course malfunctioning lenses may electrically damage camera bodies, but I'd imagine that risk is slim indeed and would not worry about that either.
Keeping a lens at 43°F indefinitely does no harm. The only issue for the camera as a whole might be the batteries, although 43°F is still pretty warm, so even those should be unscathed.
The real danger is when you remove the camera from the fridge. It will be colder than the surroundings, so moisture in the air will condense on it. Even that is clean (distilled) water, and as long as it doesn't pool shouldn't cause any long term effects after the camera has warmed and the coating of condensation has evaporated.
The thing to do is to remove the whole backpack from the fridge without opening it. Put it in the corner of the sofa, throw some towels over it, go to work, and when you come back around 8 hours later everything will be fine and ready to use normally. The point of the sofa and towels is to insulate the backpack so that the temperature of the camera inside changes only slowly. The point of not opening the backpack is to keep condensation from happening on the camera directly. Condensation, if any due to the slow warming, will happen on the outside of the backpack where it won't do any harm. 8 hours should be enough for the backpack and its contents to slowly warm to room temperature, and for whatever condensation that may have formed early on to evaporate.
43°F is not a problem as such. The problem with a fridge, however, is that it feeds air from a quite warmer room likely to contain more humidity than sustainable at fridge temperatures. You state "I'm the kind of guy that doesn't really care that there's a backpack in my fridge for a month" but it makes quite a difference whether the fridge stays shut all that while or whether you get to see the backpack all the time because of opening and closing the door. If it's the latter, the lens had a lot more opportunity to gather condensing humidity. A sort-of mitigating factor is that it would have to pass through the backpack first. That's sort-of reassuring because the backpack material tends to be a better breeding ground for fungus than the lens is on its own.
So there a number of variables in question here, and several can make a difference or not, to the degree that you are likely better off asking your lens rather than StackExchange whether you have grounds to be worried. Either way, there is not a whole lot you can do. Packing the lens with desiccant has been mentioned: this will halt any prospective bad processes but not revert whatever they might have done so far.