How to I get the timer counter to restart when the player hits the ground and respawns?

I canhed this post to rewrite your two classes a little cleaner and commented them pretty well to help you understand.

 using UnityEngine;
 using UnityEngine.UI;
 
 public class ScoreManager : MonoBehaviour
 {
     // SerializeField is a way to see the privae variable in the inspector. 
     // When coding it is a good idea to not let a variable be accessed by anything that
     // it doesn't need to be.  If it is only used in this class it should be private. 
     [SerializeField]
     Text scoreText;
     [SerializeField]
     Text hiScoreText;
     [SerializeField]
     float scoreCount = 0;
     [SerializeField]
     float hiScoreCount = 0;
     [SerializeField]
     float pointsPerSecond = 2;
    
     // Assign the player to this in the inspector. 
     [SerializeField]
     SimplePlatformScript player;
     const string sScore = "Score: ";
     const string sHighScore = "High Score: ";
 
     void Update()
     {
         if (scoreIncreasing)
             scoreCount += pointsPerSecond * Time.deltaTime;
 
         if (scoreCount > hiScoreCount)
         {
             hiScoreCount = scoreCount;
             // The high score only needs to be updated when the score is higher. Not every frame. 
             UpdateHighScoreText();
         }
         UpdateScoreText();
     }
 
     public void ResetPlayerScore()
     {
         scoreCount = 0;
         scoreText.text = "Score: 0";
      }
            
     void UpdateScoreText()
     {
         scoreText.text = string.Format("{0} {1}", sScore, Mathf.Round(scoreCount));
     }
 
     void UpdateHighScoreText()
     {
         hiScoreText.text = string.Format("{0} {1}", sHighScore, Mathf.Round(hiScoreCount));
     }
 }
 
 public class SimplePlatformScript : MonoBehaviour
 {
     //[HideInInspector] public bool facingRight = true;
     [HideInInspector] public bool jump = false;
     //public float moveForce = 365f;
     //public float maxSpeed = 5f;
     public float jumpForce = 1000f;
 
     public Transform groundCheck;
     // Store the layer in this reference so it doesnt have to create a new reference 
     // every physics frame.
     [SerializeField]
     LayerMask whatIsGround;
 
     // Changed this to a public so the ScoreManager can see it. 
     [SerializeField]
     public bool grounded = false;
 
     //private Animator anim;
     private Rigidbody2D rb2d;
     // Use const string anytime you are calling a string more than once. 
     // Everytime you call a literal string it puts a new one in memory.  
     // Doing it like this make it only create one. 
     const string sJump = "Jump";
     // Use this for initialization
     void Awake()
     {
         //anim = GetComponent<Animator>();
         rb2d = GetComponent<Rigidbody2D>();
     }
 
     void Update()
     {
         if (Input.GetButtonDown(sJump) && grounded)
         {
             // This is ok to do in update as it feels better to the player. 
             // This is a physics action but doing an input actions
             // feels off to the player if there is a delay. 
             rb2d.AddForce(new Vector2(0f, jumpForce));
         }
     }
 
     void FixedUpdate()
     {
         // Any physics calculations are better done in the FixedUpdate. 
         grounded = Physics2D.Linecast(transform.position, groundCheck.position, whatIsGround);
     }
 }

This is my player jump script. using UnityEngine; using System.Collections;

 public class SimplePlatformScript : MonoBehaviour
 {
 
     //[HideInInspector] public bool facingRight = true;
     [HideInInspector] public bool jump = false;
     //public float moveForce = 365f;
     //public float maxSpeed = 5f;
     public float jumpForce = 1000f;
     public Transform groundCheck;
 
 
     private bool grounded = false;
     //private Animator anim;
     private Rigidbody2D rb2d;
 
 
     // Use this for initialization
     void Awake()
     {
         //anim = GetComponent<Animator>();
         rb2d = GetComponent<Rigidbody2D>();
     }
 
     // Update is called once per frame
     void Update()
     {
         grounded = Physics2D.Linecast(transform.position, groundCheck.position, 1 << LayerMask.NameToLayer("Ground"));
 
         if (Input.GetButtonDown("Jump") && grounded)
         {
             jump = true;
         }
     }
 
     void FixedUpdate()
     {
         if (jump)
         {
             //anim.SetTrigger("Jump");
             rb2d.AddForce(new Vector2(0f, jumpForce));
             jump = false;
 
         }
     }
 }