Actually it can be restained. Adam is right about the "need to know what condition it is in". I will describe the procedures you will need to do.
Finish in good shape but varnish is faded (no sheen).
In this situation the stain might be faded but the varnish isn't peeling. You need start with a good washing of the doors surface. This will remove the accumulated dust and some other contaminates. Now wipe the door down with Paso. Paso is a powerful solvent based deglosser/cleaner.
Restaining will involve using a gel stain, I recommend Old Masters gel stains. You will also need a good white bristle oil brush, about 2 inches wide. Have some paint thinner and rags handy.
Matching the stain exactly isn't possible but you can go darker. Apply the gel stain with the brush to small area and wipe the excess stain out of the brush with a rag. Now dry brush, one one direction only, pulling the excess stain into a new area. Repeat thin process working in sections. Also always follow the embossed grain in the door.
Working with gel stain takes practice to get it right. Don't be surprised if you need to remove the gel stain and start over, that is what the rags and thinner is for. The stain can be removed as long as it isn't dry.
Old Masters gel stain will stick very well to the varnish and it can accept a protective varnish coat once dry, 24 hours.
Finish needs to be removed (cracked or flaking).
In this situation a paint/varnish stripper must be used. I have used Back to Nature paint strippers on fiberglass without any problems. Make sure the stripper is rated for fiberglass.
Once the varnish is removed some of the stain might be remaining. You should try to remove the bulk of the old stain for a more even look with the new stain. Once again use the Old Masters gel stain. Follow the above instructions for application and finishing.
Refinishing your fiberglass door is just like refinishing kitchen cabinets. I have written a couple of articles about this, check out cabinet refinishing for more details.