This article details how to reopen an assessment that's been closed or has passed its end date.
Click the edit button in the top right.
Click Advanced Options to open the menu of advanced options for this assessment.
In the advanced options menu, look for the scheduling options as seen below. This is where you'll need to make a change so you can re-open the assessment.
Changing the finish time to a date and time after the current date will re-open the assessment.
Click Save and the assessment will be re-opened.