[lcstorc]

Way too much missing info to make a guess. For starters what kind of clam? What is your lighting and where in the tank was the clam? What are your water paramaters and tank mates?
It would be almost impossible to make a guess without at least thse answers.
It does however seem like the clam is dead. If you no longer see a reaction to light, remove it and try to syphon out as much of the body as possible.