Hi and welcome to the forum
Your going to have to provide a bit more information about the fish and when/ where you got it from.
How long has the tank been set up for?
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Is the filter cycled/ established?
How often do you clean the filter and how do you clean it?
How long have you had the fish?
What other fish are in the tank?
What did the fish look like before and after it died?
Can you post a picture of the fish?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
The following link has information about what to do if your fish get sick. It's long and boring but worth knowing. I recommend printing it out and reading it in bed to help fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/
If you ever lose a fish or the fish look sick, test the water and do a 75% water change and gravel clean the substrate every day until the issue is resolved.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.