I had this one cichlid that suddenly died on me, and there was no visible signs of illness. The day before, it was eating fine, and pooping as per normal, but the next day I found it dead at the bottom of the tank, and my pleco feeding on it..
What size is your tank (width, height, length, gallonage)?
Did you cycle it properly?
What are your water parameters (Ammonia, Nitrite, Nitrate, pH)?
What type of Cichlid was it that died?
What other occupants are in the tank?
What have you been feeding them?