Hi Valerie, like LJ stated this is a hard one to visualize, but a few questions might help to understand the situation. The step being short... are you referring to the height (rise) of the step, or the length (run/tread) of the step? Also where is this particular step in relationship to the other steps? Is it at the bottom, top, in the middle, near a landing, etc???
If the tread is just worn out, broken making it shorter than the other treads, then it should just be a matter of replacing that tread. Now if it's something odd, say a short riser (height) in the middle of the other steps then most likely the entire stringer (set of steps) will have to be replaced. The more details that you can provide (be as specific as possible) will help determine what can be done.
Probably though the easiest thing to do is call a good carpenter and have them look at it and give you an estimate. Being able to see this firsthand obviously makes things easier to determine what steps (sorry no pun intended) need to be taken to fix this.