We opened up our two caucasian hives yesterday. One is doing great - lots of activity- bringing in pollen. Capped brood and larva. The other isn't moving at all. In fact we thought they were dead. We opened them and it appears the hive got wet. We were surprised to find a grapefruit size of bees with a queen in the top box. But no brood or larva. We cleaned up the boxes and condensed them into one box. They have honey stores still. What should our next step be with this hive aside from waiting for warmer weather as you advised Lucia?
You already did the right thing. Reducing the size of the box will make it easier for a smaller cluster to keep warm.
It's normal that they would not have brood, being a smaller cluster of bees. They need more bees to make sure they can keep the brood at about 95F.
If you have a screened bottom, make sure to have the tray in all the way. They will still need to have both, top and bottom entrances. That way they can more air as needed and prevent moisture condensation inside the hive (even though some heat is lost over the top entrance).
If the strong hive had lots of resources (food and bees) and if both hives are in the same yard, you could revert the hives (switch places) to reinforce the weak hive. The foragers would go out from the strong hive and when they come back, they'd enter the weak hive to reinforce numbers. This is only safe to do if the strong hive was not lacking any resources and would be OK while losing some foragers.
Also, if your area keeps having consistent maximum daily temperatures above 50F, you could start giving sugar syrup to your bees. This feed would stimulate your bees to brood up faster.