To reenable an inactive user, start by finding the user in the Learners page, by searching for the user in question.
Next, click on the links on either the first or last name column to view more information on this user. This will take you to the overview of this users account.
From here, next to the users name, there is a grey box that says "inactive", clicking this box will bring the drop down menu down, where you will have the option to re-activate the user.
After clicking "Re-Activate", this user will now be active again!
