It would be helpful to know what caused the stain. Every stain is different as far as what you might use to treat it.
Have you tried a Steam Cleaner? Call the professionals! Insurance Claim? Replace carpet or not! YOUR OWN HOME OR RENTED?
It would be best to know what type of stain you have in order to provide good advice about how to remove the stain. If you feel it is simply dirt, Awesome from the DollarTree applied to the area, scrubbed in well with a brush, then rinsed, rinsed, rinsed and botted up may decrease the stain quite well. Here's some advice that might be of help to you.
Hello. Do you know what the stain is, that might give us some more clues on how to offer the best solution. Very often I use the Oxi carpet cleaners that are seen on the commercial retail shelves along with my rug doctor steam cleaner Most things can be removed except dyes.
Consider posting a photo and some more information so that we can give you the absolutely best direction or suggestion
See if this helps you out
You've gotten some good advice for what to do if it's a stain that you can remove. If you can't remove it, you could try patching your carpet if you have a sample square the same color:
We like to use Capture for tough stains. We got it in the carpet area at Lowe's. Good luck!
You can rent an industrial strength carpet cleaning machine from home improvement and/or grocery stores. That will likely do the trick!
This is tough to answer without knowing what the stain is and what you’ve tried. My suggestion would be to rent a steam cleaner from your local hardware or grocery store.