Unity Rigidbody2D速度突然冻结

我用c#编写了一个使用Rigidbody2D速度的角色移动脚本。然而，有时当我尝试移动时，我的角色移动后突然冻结，无法前进。只有向后。我检查了碰撞器，它们都是相等的。我甚至尝试了AddForce，但它仍然死机。

using UnityEngine;
using System.Collections;
public class CharacterController2D : MonoBehaviour {
[SerializeField]
float speed = 5;
[SerializeField]
float jumpForce = 500;
[SerializeField]
LayerMask whatisground;
[SerializeField]
bool isGrounded = false;
Transform groundCheck;
private Rigidbody2D rb2d;
// Use this for initialization
void Start () {
    rb2d = gameObject.GetComponent<Rigidbody2D> ();
    groundCheck = gameObject.transform.GetChild (0);
}
void FixedUpdate(){
    float hor = Input.GetAxis ("Horizontal");
    rb2d.AddForce (new Vector2 (hor * speed,0));
    //rb2d.velocity = new Vector2(hor*speed,rb2d.velocity.y);
    isGrounded = Physics2D.OverlapCircle (groundCheck.position, 0.15F);
}
// Update is called once per frame
void Update () {
}
}