why the first loop doesnt stop..

[transgalactic2]

why the first loop of the function doesnt stop??

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

typedef struct node node;
struct node{
	int value;
	struct node * next;
};
 int countletters(char *str);
void main()
{
	int g;
	char * str="aabx bXcb bBxaDAa";
	g=countletters(str);
}


int countletters(char *str)
{
  char ch='a';
  char *st;
  int cnt=0;
  for(st=str;st!='\0';st++)
  {
	  if ((*str>='A')&&(*str<='Z'))
	  {
        *str+=32;  
	  }
  }
   for(;ch<='z';ch++)
   {
      for(st=str;st!='\0';st++)
	  {
           if (*st==ch)
		   {
              cnt++;
			  break;
		   }
	  }
   }
   return cnt;
}

[transgalactic2]

solved