why dose my sprite not jump but will move.

Questions & Answers
1

#pragma strict

var moveForward : KeyCode;
var moveBack : KeyCode;
var jump : KeyCode;
var speed : float = 10;
private var ground = false;

function Update ()
{
	if (Input.GetKey(moveForward))
	{
		rigidbody2D.velocity.x = speed;
	}
	else if (Input.GetKey(moveBack))
	{
		rigidbody2D.velocity.x = speed *-1;
	}
	else
	{
		rigidbody2D.velocity.x = 0;
	}
	if(Input.GetKey(jump)&& ground == false){
	  
		rigidbody2D.velocity.y = speed;
	}
	ground = true;
	
}
function OnCollisionStay()
{
	ground = false;
}
2

Your ground variable is confusing. Try this:

#pragma strict
 
var moveForward : KeyCode;
var moveBack : KeyCode;
var jump : KeyCode;
var speed : float = 10;
private var isGrounded = true;
 
function Update ()
{
	isGrounded = rigidbody2D.velocity.y == 0;

	if (Input.GetKey(moveForward))
    {
        rigidbody2D.velocity.x = speed;
    }
    else if (Input.GetKey(moveBack))
    {
        rigidbody2D.velocity.x = speed *-1;
    }
    else
    {
        rigidbody2D.velocity.x = 0;
    }

    if(Input.GetKey(jump) && isGrounded)
	{  
        rigidbody2D.velocity.y = speed;
    }     
}