Why does the array only display garbage value

You need to process only the elements that the user input, not the entire array, since the rest of the array is uninitialized.

Change initializeArray() to return the number of elements that the user input. Then use that instead of n when calling the other functions.

initializeArray() wasn't using the parameter num. It should use that to check that the user doesn't enter more elements than the array can hold. So you need to use a different variable for the parameter and the user's input, so you can compare them.

findSecondSmallest() doesn't return anything, so it should be declared void, not int.

sortAscending() is supposed to return a pointer to the array, but it was missing the return statement.

In findSecondSmallest() you can initialize the variables to the first element of the array rather than INT_MAX. Then it makes sense to start your loop from i = 1, since you've already used element 0.

#include<stdio.h>
#include<limits.h>
#include <stdlib.h>

int initializeArray(int num,int arr[]);
void findSecondSmallest(int num,int arr[]);
int *sortAscending(int num,int arr[]);
void displayArray(int num,int arr[]);

int main(void){
    int array[50],*arr;
    int n = sizeof(array) / sizeof(array[0]);
    int num = initializeArray(n,array);
    findSecondSmallest(num,array);
    arr = sortAscending(num,array);
    displayArray(num,arr);
}

int initializeArray(int n,int arr[]){
    int i, num;
    printf("Input the number of elements in an array: ");
    scanf("%d", &num);
    if (num > n) {
        printf("That's too many elements\n");
        exit(1);
    }
    printf("\nEnter %d elements: ", num);
    for(i = 0;i < num;i++){
        scanf("%d", &arr[i]);
    }
    return num;
}

void findSecondSmallest(int num,int arr[]){
    int i,secondSmall,small;
    small = secondSmall = arr[0];
    for(i = 1;i < num;i++){
        if(arr[i] < small){
            secondSmall = small;
            small = arr[i];
        }else if(arr[i] < secondSmall && arr[i] > small){
            secondSmall = arr[i];
        }
    }
    printf("The second smallest element is %d\n", secondSmall);
}


int *sortAscending(int num,int arr[]){
    int i,j,temp;
    for(i = 0;i < num;i++){
        for(j = i + 1;j < num;j++){
            if(arr[i] > arr[j]){
                temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }
    }
    return arr;
}

void displayArray(int num,int arr[]){
    int i;
    for(i = 0;i < num;i++){
        printf("Element at Array[%d] = %d\n", i, arr[i]);
    }
}