You can reactivate a user at anytime, you may want to do this if someone was deactivated in error or has rejoined your company.
Finding a deactivated user
Click the grey cog icon at the top right corner of the page and select Administration centre. Then enter the People tab.
Find the deactivated user in the People list by filtering for Deactivated only users.
Once you have found the correct person, click on Edit located under the Edit column. Then click the orange Activate this user button.