This same thing happened to me just the other day... well, not me, but my Maxima. I noticed a large chunk of tissue and the clam was on its side. I looked under the clam and could see a bit of tissue still inside. it took several days for it to reattach, but once it was happy it reattached no problem.
They can drop the byssal threads at will I guess and that does not mean that they have lost the byssal gland. So, make sure you give us an update on how yours is doing.