If it detached itself, then it may just not have liked the spot......
The best way to deal with 2" clams is to put a small rock on your sandbed, then try to get the clam to attached to that rock....perhaps put a pvc tube to perch the clam on with the rock inside of the tube. To spot feed, cut the top off of a 2L bottle. Cover the clam with the bottle, and shoot some phytoplankton in and set the cap on the opening.....