Metrosideros 'Red and Gold' in a terrarium, why has it suddenly started to droop?

About a month ago, I created an enclosed terrarium with a Metrosideros (New Zealand Christmas Tree) recommended by both the local garden center and the grower, and followed the instructions from a blog to create the terrarium. I put it in a partially shady part of the kitchen, and lifted the lid once when condensation appeared. Then all of a sudden yesterday half the plant drooped. I gave it a small drink of water (like a 1/4 cup), but this morning the other side of the plant was dropping as well. I was wondering if anyone has any idea what happened? Or if anyone has general info for taking care of this plant? The grower has conveniently taken it off their website, and the general gardening sites have been no use. Thanks!