

If you have caps to spare, make sure all of your power rooms are triple-wide. If those dwellers were in the Power room, it's possible that power output was not at capacity, and with a growing vault, that can easily lead to catastrophe. Unhappiness (which results in loss of productivity) can come from a variety of things, and one of those things is working next to dead dwellers. Thankfully, dwellers don't seem to die from lack of water or food. Radiation in the vault comes from lack of Water, and Sickness (low HP) comes from lack of Food.

Is it possible that you ran out of power? When the power goes out, other rooms (such as the cafeteria and the water treatment plant) risk going offline, and when they power down, they don't produce resources, regardless of how many people are in them.
