When you open the job section of the employee profile of the terminated employee, you will find the Activate Employment button when you scroll to the bottom of the screen. By clicking on that button you will be able to rehire the terminated employee in the system. But as per security reasons, even though the employee is activated, the user account for the terminated employee will be in the disabled state. Therefore, we need to enable the user account manually.
Note - If you are filtering the employee, make sure the status in the filter criteria is selected as All.