2D C# Rotation Stopped Working Since Unity 5, Please Help?

Hello all, can anyone please tell me why my 2D rotation feature that I use on a sprite in my game has stopped working since I've updated to Unity 5?

Here is the code, I've removed all other elements that are used for movement up and down etc. I believe this is all that is needed to work out the problem.

 using UnityEngine;
 using System.Collections;
 
 public class PlayerControl : MonoBehaviour {
 
 float maxMove=2f; // speed of the movement of the object
 float maxTurn=50;   // max angle it turns up/down
 float maxGrav=500;
 float turnRate=10;
 
 Rigidbody2D rb2d;
 
     void Start() {
         
         rb2d = GetComponent<Rigidbody2D> ();
     }
 
     void Update() {
         float delta = 0;
         float deltaGrav = 0f;
 
         if (Input.GetKey (KeyCode.UpArrow)) {
             
             delta++;
 
         } else if (Input.GetKey (KeyCode.DownArrow)) {
 
             delta--;
 
         } else {
             deltaGrav--;
         }
 
         rb2d.rotation = Mathf.Lerp (rb2d.rotation, maxGrav * deltaGrav, turnRate * Time.deltaTime);
         
         rb2d.MovePosition (rb2d.position + Vector2.up * delta * maxMove * Time.deltaTime);
         
         rb2d.rotation = Mathf.Lerp (rb2d.rotation, maxTurn * delta, turnRate * Time.deltaTime);
 
     }
 }

Thank you all in advanced, I really appreciate the help!