Do an immediate water change - 50%. Make sure to use dechlorinator and match the temperature of the replacement water to the tank water.
After the water change, try to pinpoint the problem - account for all the fish. Is it possible that some died a day or two ago? That can cause an ammonia spike and affect the remaining fish.
Is the heater working properly - temperature not too hot or cold? Is the filter running - nothing is clogging it?
How old is the tank and is it cycled? What size tank is it and how many fish? How did the fish look after your last water change?
If you have test kits, post ammonia, nitrite, and nitrate readings.