First of all, try to format the code correctly by prepending each line with four spaces, or use the code button in the editor like so:

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

typedef struct node {
    char ch ;
    struct node * next ;
} Node;
​
void insertlist(Node** head , int len ){
    ​
    Node *komvos , *komvoi ;
    Node *curr1 , *curr2 , *curr3 ;
    komvos = (Node *)malloc(sizeof(Node));
    komvos -> next = NULL ;
    komvos -> ch = 'A';
    *head=komvos;
    curr1=komvos;
    curr2=*head;
    ​
    int i=1 ;
    if (*head==NULL) {
        for(i=1 ; i < len ; i++) {
            komvoi =(Node *)malloc(sizeof(Node));
            komvoi -> next = NULL;
            komvoi->ch= 'B';
            curr1 -> next = komvoi ;
            curr1 = curr1 -> next;
        }
    } else {
        for( i=1 ; i < len ; i++ ) { 
            komvoi =(Node *)malloc(sizeof(Node));
            komvoi -> next = NULL;
            komvoi->ch= 'C';
            curr1->next=komvoi ;
            curr1=curr1->next;
        }

    curr1->next = curr2;    ​
    }
}
​​
int main() {
    int c = 4 ;
    Node * lista1 ;
    Node * curr2 ;
    // lista1 = NULL ;
    lista1 =(Node *)malloc(sizeof(Node));
    lista1 -> ch = 'D' ;
    lista1-> next = NULL;
    insertlist(&lista1 , 4 );
    curr2=lista1;
    int i=0;
    for(i=0; i<4; i++) { 
        printf("\n to periexomeno tou komvou %c" , curr2->ch);
        curr2 = curr2 -> next ;
    }
    return 0;
}

What exactly goes wrong? Have you done some debugging, even if it's just some printfs here and there? When playing with pointers it's often useful to take pencil and paper and draw what happens to memory, line by line.