If successful collision reverse rotation of object

How can I code a script so that if a collision is successful the controller reverses the rotation of the player controller. This is the code: using UnityEngine; using System.Collections;

 public class PlayerControl : MonoBehaviour
 {
     public float turnAngle = 90;
     public float turnSpeed = 40;
 
     private float angle;
     private Transform T;
 
     void Awake()
     {
         this.enabled = false;   
     }
 
 
     void Start()
     {
 
         T = GetComponent<Transform>();
     }
 
 
     void Update()
     {
        
         if (Input.GetMouseButtonDown(0))
             angle += -turnAngle;
 
         
          T.rotation = Quaternion.Slerp(T.rotation, Quaternion.Euler(0, 0, angle), Time.deltaTime * turnSpeed);
     }