This is a tricky situational question asked during your interview. Here the recruiter is interested to know two things. One that what measures can you take to retain a good resource and other how will you manage the loss, will you get panic. So the perfect answer could be:
I would talk to the employee and understand the root cause of such a decision. If the reason is quite genuine and personal, I will ask him to take an extended leave, and if it makes no sense to the employee, I won’t force him/her to stay back. But if the reason is something related to growth/dissatisfaction; I will speak to HR and try to retain the employee in all possible ways. By any chance, if nothing works, I will look for the next available resource and help him/her with the right training.