Let say you deactivated account in liferay, it won't be visible in Users by default. And if you try to re-register it will give error that this user already exists. Hence in order to activate the deactivated account do following:
Go to Control Panel > Users & Organizations > All Users
On right hand side select Inactive in Show drop-down.
And for given record right click Actions > Activate.
This will activate the given user. Please view attach image for more details.