Retstart Key in C# or Javascript

What you need to know about is :

• Input.GetKeyDown to detect if a key was pressed

• SceneManager.GetActiveScene to know which scene is currently playing

• SceneManager.LoadScene to load a scene

• Scenes in Build Settings to add a scene to the Build Settings. At runtime, you can only load scenes that have been added to the Build Settings.

• DontDestroyOnLoad to tell Unity not to destroy an object when loading scenes

So you just have to write a script that will reload the current scene when a certain key is pressed, and doesn't get destroyed when loading a scene. Then you attach this script to a GameObject in your first scene, so that it is accessible in every scene of your game.

   using UnityEngine;
   using UnityEngine.SceneManagement;
 
   public class ReloadLevel : MonoBehaviour
   {
     // This function is only called once, when the object is initialized
     void Awake()
     {
         // Tell Unity not to destroy the GameObject this script is attached to
         // when loading a scene. That way, you only need to place an object
         // with this script in your first scene, and it will stay in every
         // scene.
         DontDestroyOnLoad(gameObject);
     }

     // This function is automatically called every frame
     void Update()
     {
         // Detect if R or Return were pressed
         if (Input.GetKeyDown(KeyCode.R) || Input.GetKeyDown(KeyCode.Return))
         {
             // Get the active scene
             Scene currentScene = SceneManager.GetActiveScene();
             
             // To reload this scene, we have to use its name or its build index
             // so that the Scene Manager knows what scene it needs to load.
             // Get the Build Index of the active scene
             int buildIndex = currentScene.buildIndex;
             
             // Tell the Scene Mmanager to reload the scene corresponding to this
             // Build Index in the Build Settings
             SceneManager.LoadScene(buildIndex);
         }
     }
 }

And voilà, it's actually pretty easy, right ?