What was it like prior to discovering it dead? Was the coloring vibrant? Was it responsive (aka closing quickly under a passing shadow or when nudged), was it gaping irregularly, was the mantle tissue extended?
All of these could have been warning signs that something was wrong days before the unfortunate discovery. There's also a phenomenon called "mysterious clam death" where death can come suddenly with no apparent cause in clams.
A few things that may have gone undetected are certain types of very tiny snails may have infected it, a microbial outbreak that you normally wouldn't tell is even happening until something like this happened, poor lighting (may have been ongoing if you weren't aware of the warning signs mentioned above) or even worms.
Sorry for your loss :(