WAS the oldest bloom the last to die, or is it that the spike has lost all the flowers and is yellowing from the tip? Once blooms drop most spikes do die back from the tip. If you want to try for a rebloom, cut the stalk now immediately above the first (highest) node that did not have a blossom.
It may work, it may not. I was fully open when you got it? Could be it just wasn't happy with the change and decided to 'sulk' for a while. Leaves look okay?