If you deactivated a user you can reactivate them again if needed which will allow them to sign back into the system:
Top right, click your name and select Users. *If you do not see "Users" you may not have access to get to his area of the program.
Bottom left, change the "Show" from Active Users to All Users.
Select the specific User and click Edit
Change their Status from Disabled to Active and click Save.
You can then adjust their permissions and client level access if needed from here as well.