As mentioned, use OnCollisionEnter instead. Also, I have OCD and your code irked me so I had to make it pretty. Note: I removed your While statement as it was being used oddly.

using UnityEngine;

public class PlayerController : MonoBehaviour {

    public float speed;
    public float jumpHeight;
    private Rigidbody2D rb;
    private bool isGrounded;


    void Start () {
        rb = GetComponent<Rigidbody2D>();
    }

    void Update () {
        if (isGrounded && Input.GetKeyDown(KeyCode.W)) {
            Vector2 jump = new Vector2(0.0f, jumpHeight);
            rb.AddForce(jump, ForceMode2D.Impulse);
        }
    }

    void FixedUpdate () {
        float move = Input.GetAxis("Horizontal");
        Vector2 movement = new Vector2(move, 0.0f);

        rb.AddForce(movement * speed);
    }

    void OnCollisionEnter (Collision coll) {
        isGrounded = true;
    }

    void OnCollisionExit (Collision coll) {
        isGrounded = false;
    }
}