Related Questions:
Can I re-activate a User?
How do I reinstate someone who used to be a User?
Is it possible to reactivate an old User?
Answer:
You can re-activate someone who used to be a User and has been deactivated. This can be done by following these steps by an Admin User.
Click Settings
Choose Organization
Click on Users & Roles
Toggle the list to Show Deactivated
Loccate the User you would like to Re-activate
Click the Activate Icon