My program stops working during execution

I write my code in code blocks and I get this runtime error during execution:

Process returned (0xC0000005)

This is my code

I believe it has something to do with the way I call my functions

main:

int main()
{
    int LT[101],i, NI;
    float x;
for (i=0 ; i<101 ; i++)
LT[i]=i*i;
NI=find_NI(LT , x);
x=recieve_check_x();
cout<<"square root  of x is:"<<(NI+(x-LT[NI])/(2*NI));
return 0;
}

rec_chk.cpp:

#include <iostream>
#include "funcs_hd.h"
using namespace std;
float recieve_check_x ()
{
    float x;
cout<<"enter the value of x:"<<endl;
cin>>x;
while (x<0 or x>10000)
    {
    cout<<"\a error! x must be within range of [0,10000]"<<endl;
    cout<<"enter another value:"<<endl;
    cin>>x;

    }

    }

find_NI:

#include <iostream>
#include "funcs_hd.h"
#include <cstdlib>
#include <cmath>
using namespace std;
 int find_NI (int LT[] , float x)
 {
 int i, j , NI;
 j=i+1;
 double i_delta=abs(LT[i]-x);
 double j_delta=abs(LT[j]-x);
 if (i_delta<j_delta)
    NI=LT[i];
    else
    NI=LT[j];
    return NI;
 }

funcs_hd:

#ifndef FUNCS_HD_H_INCLUDED
#define FUNCS_HD_H_INCLUDED

float recieve_check_x (void);
int find_NI (int [] , float);
#endif // FUNCS_HD_H_INCLUDED

I get an error code and the program stops running

int i, j , NI;
j=i+1;
  ^ indeterminate value

You read an indeterminate value in the program. Therefore the behaviour of the program is undefined.