Hi
Milk teeth starts to fall out approximately from the age of 6 and continue all the way till age of 13. So, if the tooth that fell has fallen earlier than it should have fallen, it would take some time for the permanent tooth to come out. Since you have not mentioned which tooth has fallen, either you can wait till the tooth erupts or you can consult a Pediatric dentist.
Next Steps
Visit a near by Pediatric dentist.