I am doing the raytracer in a week and i only render the first row.How can i fix it?

You do not initialize ncount back to zero. After the first loop, ncount is always equal to WIDHT (256), skipping the inner loop body. You should learn to step through your code in a debugger, which will allow you to see the variables at any point.

fn main() {
    const HEIGHT:i32= 256;
    const WIDHT:i32 = 256;

    println!("P3\n{} {}\n255",WIDHT,HEIGHT);

    let mut i_count=HEIGHT-1;
    let mut n_count=0;

    while i_count >= 0
    {
        n_count = 0;
        while n_count <=WIDHT-1
        {
            let r = n_count as f64 / (WIDHT-1)as f64;
            let g = i_count as f64 / (HEIGHT-1)as f64;
            let b =0.25;

            let ir = (255.999*r)as i64;
            let ig = (255.999*g)as i64;
            let ib = (255.999*b)as i64;

            println!("{} {} {}",ir,ig,ib);
            n_count=n_count+1;
        }
        i_count=i_count-1;
    }
}

What @Dark said.

As an additional remark, the exact problem you ran into is the reason why this kind of loop is discouraged (not just in Rust, but in general). If you want to iterate through a range, use range iterators instead. They are much less error prone.

fn main() {
    const HEIGHT: i32 = 256;
    const WIDTH: i32 = 256;

    println!("P3\n{} {}\n255", WIDTH, HEIGHT);

    for i_count in (0..HEIGHT).rev() {
        for n_count in 0..WIDTH {
            let r = n_count as f64 / (WIDTH - 1) as f64;
            let g = i_count as f64 / (HEIGHT - 1) as f64;
            let b = 0.25;

            let ir = (255.999 * r) as i64;
            let ig = (255.999 * g) as i64;
            let ib = (255.999 * b) as i64;

            println!("{} {} {}", ir, ig, ib);
        }
    }
}