Assuming the following (same as paul):

the old prof is gone for good
there are no available grades in the system
the exams went with the prof, and possibly destroyed
we are talking about last fall's term, so the course has finished and the students have moved on

If the class isn't absolutely vital, assigning all students who weren't failing going into the class "Credit" might be acceptable. You can get that information from student's copies of previous assignments returned. If the former professor isn't answering, the wheels of justice turn so slowly you won't get them back in time.

Assuming the following

• the old prof is gone for good;
• there are no available grades in the system;
• the exams went with the prof, and have possibly been destroyed;
• we are talking about last fall’s term, so the course has finished and the students have moved on.

If the class isn’t absolutely vital, assigning all students who weren’t failing going into the class “Credit” might be acceptable. You can get that information from students’ copies of previous assignments returned. If the former professor isn’t answering, the wheels of justice turn so slowly that you won’t get them back in time.