There is no code that let the program stop anywhere after you have entered the two numbers.
If you want to let it run for another calculation, use a while loop until a specific condition is met (both numbers null here):
main()
{
int a,b,sum;
do
{
printf("\nType your frist number: ");
scanf("%d" , &a);
printf("\nType your second number: ");
scanf("%d", &b);
sum=a+b;
printf("\n The sum of two number you have just type is %d", sum);
printf("\nEnter null for both numbers to terminate");
}
while (a != 0 || b != 0);
}