you have not covered the condition where the head is null. if you want to do this in this manner , your code should be somthing like this:

Node* Insert(Node *head,int data) { if(head==NULL) { Node *newn=(Node *)malloc(sizeof(Node));

        newn->next=NULL;
        newn->data=data;
        head=newn;

    }
else{
     Node *newn2=(Node *)malloc(sizeof(Node));
    newn2=head;
    while(newn2->next!=NULL){
        newn2=newn2->next;
    }
    newn2->next=Insert(newn2->next , data);

}
return head;

}