To change the hidden status, look at the activity on your course page. From the edit menu, select Show.
Your preassessment clearly shows that only one student has completed it. Are the other student's attempts still showing under the preassessment and the quizzes? The fact that two are showing as completed indicates that the completion setting must be working.
Where is your Moodle hosted? Is it possible that a backup was restored that erased the previous attempts?