#include <math.h>
#include <iostream>
#include <iomanip>
using namespace std;

int main()
{
        __int64 a;
    int i=0;
    double b[32768];
    while (cin>>a)
    {
        b[i]=sqrt(double(a));
        i++;
    }
        i--;
    for (int k=i; k>-1; k--)
        cout<<fixed<<setprecision(4)<<b[k]<<'\n';
        return 0;
}

Increase the size of array!

I've already tryed to increase to 100000 and got the same error...

make your array global.
Or use linker directive increasing stack size - the instructions are in FAQ.

increase it to 150,000 !!!