Follow the steps below to enable an inactive/deactivated employee:
Click Settings->Employees
Toward the top of the list click the status dropdown and select "Inactive"
Find and click on the employee's name to access their profile
Click on the Admin tab
Click on Edit Employee (just below the Admin tab)
After clicking on Edit Employee, a button labeled 'Restore Employee' will become visible. Click on this button and a prompt will pop up asking if you're sure. Click Yes.