UI: 'GetComponent' not showing up in Inspector.

Hello,

I'm trying to build a Game Over Screen with the newer UI, and I followed the same steps that I used while setting up my Title Menu, but for some reason I'm having an issue. In the Inspector where the 'GetComponent' will usually have spaces to drag in buttons and a canvas, there is just a space displaying the 'GameOver' Script.

Here's the one I have that's working, for reference:

And here is the one that I'm having issues with:

I'm really not to great with code, but perhaps I haven't told my script to expose them to the inspector? If I've done something completely wrong in my code, please let me know so I can improve! Here is a copy of my code:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class GameOverUI : MonoBehaviour {
 
     int score = 0;
     private bool retry;
 
     public Canvas gameOver;
     public Button retryLevel;
     public Button returnMain;
 
    
     void Start ()
     {
         score = PlayerPrefs.GetInt("Score");
         gameOver = gameOver.GetComponent<Canvas>();
         retryLevel = retryLevel.GetComponent<Button>();
         returnMain = returnMain.GetComponent<Button>();
         gameOver.enabled = true;
     }
     
     public void RetryLevel()
     {
         Application.LoadLevel(1);                    //Level that is my game
     }
     public void ReturnToMainMenu()
     {
         Application.LoadLevel(0);                    //Level that is the Title Menu
     }
 }
 

Also, you will probably see I have a variable declared for 'score' that isn't used in the code. I have a high score that grows based on time that's passed, and amount of power-ups picked up, and would like to add that to my 'Game Over' screen, but am not sure how. Any advice there would be great also!