If it's a Moth Orchid, a Phaelaenopsis, then wait until the last flower falls off the stem, then cut the stem off just above the bottom node - the little scale on the stem. Then, in time, it should produce a new stem, either from the node, or from between the leaves.

If it's a Dendrobium, Paphiopedilun or Oncidium type cut and paste the link below to your browser.

http://www.growsonyou.com/question/show/33927