She wasn't dead. Prior to being married, Koh grabbed Ummi and took her into the Spirit World. She was still alive, however. Kuruk became outraged due to losing Ummi. That is why he comes back to Koh's lair annually in attempts to find her, though failing in every occasion angers him further. She didn't die and go to the Spirit World, her physical body was brought there by Koh.
However it's not entirely clear whether this fact is true as the series never really gives the exact circumstances behind Koh's attack on Ummi. Whether she was living or not at the time it is implied that when Koh steals someone's face he takes their identity and their ability to guide themselves in the spirit world. That once Koh has stolen a person's face he takes away their ability to find eternal peace, think of the monkey spirit entity that Aang encounters before meeting with Koh.