Sometimes an employee may leave employment with your company and then return to work as a new employee.
Once an employee has been terminated, to re-instate them a new record needs to be created. To create a new employee record you can follow the instructions in this article.
The reason is that when an employee is terminated and this is reported to HMRC, they need to be setup as a new payroll record (with a new payroll Id) with HMRC otherwise it can cause issues with HMRC's RTI reporting.