If the Individual record has a Portal tab, click on the Portal tab then click Reactivate Link.  This will send an email to the portal user with a link to activate the account.  If the Individual record does not have a Portal tab, you will need to reactivate the portal user's account.  From the Options menu, click Users under Manage Tenant Settings.  Scroll to the bottom of the screen and click View Portal Users.  Scroll to the bottom then click View Inactive Portal Users.  Find the portal user's name and click Reactivate Portal User.