Any users that were disabled can be reactivated.
You can activate a user by searching for disabled users in the User panel search box.
To search for deactivated users:
1. Click the search icon (magnifying glass) at the top right corner of Users panel:
2. Switch status to Non-active users and click the Search button:
3. Click on the user icon and choose 'Activate':
If you want learn how to disable a user click here.