Shimano ratcheting shifters (STI and Rapidfire) sometimes have an issue of the internal lubrication - a translucent green grease - getting hard over years of use, esp. after a few seasons of freeze-thaw cycles, and occasionally just when the weather gets cold. This might be the issue here - the internal workings of the shift lever are gummed up in the cold.
It could also be that some moisture got into the shifter, a segment of cable housing, or into the derailleur, and this froze. I left my fat bike out in the weather last week and when I went to use it there was no shifting. I removed the bottom cover off the shifter and sat it in front of the fireplace to dry out and it works fine now, The same effect would result from moisture in the cable housing. I think it's less likely that the derailleur itself will be affected by this, but it's possible.