Howdy DeborahD,
The doors should be removed from the track so you can see how the rollers are adjusted up or down on the top of the shower doorframe. This is also to get a better idea of what to fix the damaged roller.
The rollers are made of a nylon type material that can chip or crack over time and if any of the rollers show any sign of such wear, it usually makes sense to replace them all.
Take one roller off and take it with you to your local Home Depot to see about matching the specific size of your rollers. Remember, they do not have to look like the old ones, just have the proportions that will work for you.
If they are in bad shape, you might have to think about replacing them all together. Sometimes the amount of work is more than what new doors cost.
Happy Fixing,
Coach Dave