Assessments can be reopened after their end date so that students who did not complete the assessment in the allotted time can have a second chance.
Go to the Assessments section of your Control Panel.
Hover over the assessment you wish to reopen and click Details.
Once you are viewing the details of your assessment, you will see the Re-open Assessment option.
Upon clicking this option you will be prompted to set a new due date for the assessment. You will also have the option to Count any new work done as late. This can be a good way to distinguish between students who completed their work on time on those who didn't.
Once you are happy with the new details of the assessment, click Submit. Upon doing so, you will be returned to the Details section of the assessment. There will now be a note under the Start & Due Dates heading that will let you know the new deadline for the assessment.