Remove the dead plant from the rest of the arrangement right away. The dead roots can spread rot to the other plants if left in the water too long. Remove the entire arrangement from the water. Take the dead stalk out by getting a firm grip on the stalk just above the roots and slowly pulling in out. If the plant is tied into the arrangement then loosen the wires by untwisting them and pulling the dead stalk out. You may need to gently pull the roots apart from the other plants if they are entangled. Don’t worry if you end up ripping some roots, as these should quickly regenerate. If the arrangement is made up of many braided strands removing one dead stalk shouldn’t hurt the overall strength of the arrangement.