Uzzah died after reaching out to touch what?

Answer: The ark of God

When the oxen stumbled, Uzzah took hold of the ark on its new cart, and God struck him down for his irreverence (KJV: error).

“They set the ark of God on a new cart and brought it from the house of Abinadab, which was on the hill. Uzzah and Ahio, the sons of Abinadab, were guiding the new cart,”— 2 Samuel 6:3 (BSB)