As a department head, what to do if a professor leaves without notice without giving out grades?

I am department head, and an adjunct professor teaching two courses in my department left without notice. He did not delivered the grades to me.

We have no staff in that topic to take the final exam again. Even so, I do not know how to convince the students to take the exam again, and I do not know what to do with the class activity, which should be part of the final grade.

Registrar is forcing me to deliver the grade, but I have no idea what to do.

Any experience to share? What do department heads normally do in this situation?