Dead fics are fics that are uncompleted but no longer updated. Lost fics are fics that are (presumed to be) no longer on the internet.
Most dead fics become lost fics, most lost fics are dead fics, but there's plenty of completed fics that become lost also.
Most dead fics become lost fics, most lost fics are dead fics, but there's plenty of completed fics that become lost also.