Picture and video of the fish?
Upload video to YouTube, then copy & paste the link here.
If you use a mobile phone to film the fish, hold it landscape mode.
What does the fish's poop look like?
When did it stop eating?
Did you add or do anything to the tank in the 2 weeks before this happened?
What is the ammonia, nitrite, nitrate and pH of the water?
What sort of filter is on the tank?
How often and how do you clean the filter?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
Assuming the tank and filter are clean, it could be an internal bacterial infection but you need to see what its poop looks like. Stringy white poop and not eating anything is an internal bacterial infection and antibiotics are required.