First guess is that it got very stressed by the "little water change". Perhaps it made the water conditions very different too fast. Please provide some more information.
How much water did you change? Did you dechlorinate the water before adding it? What is the PH of the water added?
What is the PH, temp, Ammonia, Nitrite, Nitrate of the water? What kind of filter? How old is the tank?
If you don't have a test kit and don't want to buy one then you might want to take a bag of water to your LFS for them to test for you.
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