Cut the stem back to just above the lowest node (flap) on the stem, if the bottom part is still green. You should see a new stem appearing within a few weeks, either from the node, or from between the leaves. (If the whole stem is brown, cut it all off)

If a new stem doesn't appear, take the plant into another room which is 5 - 8 degrees C. cooler than its normal temperature, and leave it there for 3 weeks. This 'shocks' the plant into producing a new flower stem.

Good luck.