If you've set an employee's leaving date but they are no longer leaving, you can simply remove the scheduled archiving date from their profile. This will prevent their account from being archived automatically.
Remove a scheduled archiving date
Go to HR > Employees
Select the employee
Click the Profile option
Scroll down to the Employment section
Select the Edit option
Click the 'x' in the Automatic archiving date field
Click Save
The employee will no longer be archived automatically and their account will remain active.