UPDATE: Necropsy report came back. She had chronic heart failure (bad heart valve) on top of multi-organ failure that had been brewing for some time, she just got pushed over the edge by the recent cold snap. By taking her back outside after her night in the basement, I probably sealed her fate, but on the bright side, she was already crashing so at least it put a quicker end to her suffering. The cause of the multi-organ failure could be genetic or environmental. I had described her living conditions in detail, and the report stated that they sounded optimal, but just to rule any potential toxicity out, they want to run some tests on the tissues, so I'll have them do that. These chickens don't leave their brand new coop and run, and have no access to sketchy chemicals, lead paint or whatever... so my bet is on genetic... but we'll see. She was an assisted hatch, and was very sexually immature (as observed by me based on comb/wattles, and as confirmed by necropsy of her reproductive organs). My poor baby, sounds like she had a lot wrong with her, and it doesn't look like I could have saved her

Up until the day before her death though, she never showed any signs of trouble at all. Healthy, active, loving and awesome chicken until suddenly she was not...
I'll post an update once I get the test results back. Then I'm going to email her breeder.