Alright, I have a complete script but the objects only move down?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Bat_2_Controller : MonoBehaviour{
     private Rigidbody2D rb;
 
 
 
 
     // Use this for initialization
     void Start () {
 
         rb = this.GetComponent<Rigidbody2D> ();
     }
     // Update is called once per frame
     void Update () {
 
         if (Input.GetKey(KeyCode.UpArrow)) {
             rb.velocity = new Vector2 (0f, 7f);
 
 
         } if (Input.GetKey(KeyCode.DownArrow)) {
             rb.velocity = new Vector2 (0f, -7f);
         } else {
             rb.velocity = new Vector2 (0f, 0f);    
         }
     }
 }

Ok So here's the code, and every time I hit play it the bats can move down and when I hit up it won't budge! I have the exact same script as my friend and their game works perfectly. Anything you guys can do to help? Thanks!