When a student has an interrupt to their connection while taking a non-timed test element, the test will stop at that point. When the student re-enters that test element it will begin at the question where they were disconnected. However, you have the Forward Only option enabled on the test, the test will resume at the next question in the list.
If the test or questions are timed the clock will continue to count down to expiration. If this happens the test may need to be reset for the student by a course administrator or an offering facilitator.