What's wrong with my droopy plant?

For comparison, I've attached two images. The one that is drooping was kicking ass before and was the more upright of the two. I'm not overwatering; if anything I underwatered this plant a bit over the past few days (could it be that?). Both plants are two weeks old. This plant's growth seems to have declined over the past 24 hours.

Root-bound which explains why it was drying up so quickly. Transplanted both and watered 'em, follow my journal to see how it goes...