there is an error in finding the array is sorted or not

#include <stdio.h>

int ret(int[], int);

int main() {
    int n;

    scanf("%d", & n);

    int i;
    int a[n];

    for (i = 0; i < n; i++) {
        scanf("%d", & a[i]);
    }
    
    ret(a, n);

    return 0;
}

int ret(int a[], int n) {
    int i, c1 = 0, c2 = 0;

    if (a[n - 1] >= a[n - 2] && a[0] <= a[1]) {
        for (i = 0; i < n - 1; i++) {
            if (a[i] <= a[i + 1]) {
                c1++;
            }
        }

        if (c1 == n - 1) {
            puts("SORTED");
        } else {
            puts("NOTSORTED");
        }
    } else if (a[n - 1] <= a[n - 2] && a[0] >= a[1]) {
        for (i = 0; i < n - 1; i++) {
            if (a[i] >= a[i + 1]) {
                c2++;
            }
        }

        if (c2 == n - 1) {
            puts("SORTED");
        } else {
            puts("NOT SORTED");
        }
    }
    
    return 0;
}

I want to check whether the array is sorted or not, but I can't get the output. Sometimes it gives the correct output, but sometimes it doesn't give any output. Please check the code.

If this if statement doesn't fire if(a[n-1]>=a[n-2]&&a[0]<=a[1]) and this one doesn't either
else if(a[n-1]<=a[n-2] &&a[0]>=a[1]){, then there is no line of code that can produce any output.

As pointed out by @DavidSchwartz, sampling both ends of the array does not account for the direction of the entire array and may even preclude any 'analysis'.

Apart from that, int a[1] = { 42 }; is a perfectly valid array that would lead your code into undefined behaviour (accessing memory that is not part of the array.)

Try the following:

#include <stdio.h> 

void ret( int a[], int n ) {
	int cUP = 0, cDN =0; // Tally up/down variations.

	for( int i = 1; i < n; i++ ) { // NB: start at 1 and check backward
		cUP += ( a[ i-1 ] < a[ i ] ); // going UP
		cDN += ( a[ i-1 ] > a[ i ] ); // going DOWN
	}

    // If both up & down found, array is not sorted.
	if( cUP && cDN )
		puts( "NOT SORTED" );
	else
		puts( "SORTED" );
}

int main() {
	int a[] = { 1, 2, 3 };
	ret( a, sizeof a/sizeof a[0] );

	int b[] = { 3, 2, 1 };
	ret( b, sizeof b/sizeof b[0] );

	int c[] = { 3, 2, 5 };
	ret( c, sizeof c/sizeof c[0] );

	int d[] = { 3, 2, 2, 2, 1 };
	ret( d, sizeof d/sizeof d[0] );

	int e[] = { 2, 2, 2 };
	ret( e, sizeof e/sizeof e[0] );

	return 0;
}

In fact, the loop could bail out early with the following:

	for( int i = 1; i < n && (!cUP || !cDN); i++ ) {

If both counters become non-zero, the array is not sorted and the loop can finish early.