Student Account Disappeared. Reentered the class and lost all previous work

I had a student mysteriously disappear from the class list this morning.  His screen showed me that he needed to add the class code to enter and when we did that, it was as though he had never been in the class in the first place.  All of his assignments from the past two months are gone and we use this platform weekly.  Now, we have no way to retrieve his responses and the work that he already submitted.  

To prevent this in the future, I need a way to export data from the students.  Also, I need to figure out how this student could have found himself removed from the class and then why his information is lost.