Start timer on button press

Hey guys, I have a countdown timer that works but I want it to only start when the player presses the space button. I also want the timer to reset when the player dies. When the player dies he/she re-spawns back at the start of the level but timer does not reset. Could anyone help me? Thanks :)

using UnityEngine; using System.Collections;

public class CountdownTimer : MonoBehaviour {

 public GUIText _TimeText;
 private float timer = 60f;
 public float addTime = 10;

 public GUIStyle customGuiStyle;

 void Start(){



 }
 void OnGUI() {
     int minutes = Mathf.FloorToInt(timer / 60F);
     int seconds = Mathf.FloorToInt(timer - minutes * 60);
     
     string niceTime = string.Format("{0:0}:{1:00}", minutes, seconds);
     
     GUI.color = Color.yellow;
     GUI.Label(new Rect(850,10,250,100), niceTime, customGuiStyle);
 }
 
 
 void Update(){
     timer -= Time.deltaTime;
     
     
     
     if (timer <= 0) {
         Application.LoadLevel("Level 3 Incomplete");
     }
     
     
     
 }

 void OnTriggerEnter(Collider other)
 {
     if(other.gameObject.tag == "PickUp")
     {
         
         
         Destroy(other.gameObject);
         timer += addTime; 
         
         
     }
 }

}