Script Problem

I'm making a basic flight script for an assignment, but I'm having a huge problem. I can get the object its applied on moving, but if it stops it won't start again. it just stops and doesn't do anything.

Here's the script

var YawSpeed = 15.0;
var TurnSpeed = 15.0;
var ElevateSpeed = 5.0;
var Acceleration = 0.2;
var Deceleration = 0.3;
var MaxSpeed = 20.0;
private var Speed = 0.0;


function Update ()

{
if(Speed > MaxSpeed)
{
Speed = MaxSpeed;
}
if (Speed < 0.0)
{
Speed = 0;
}

if(Speed <= (MaxSpeed/3))
{
rigidbody.useGravity = true;
}

else
{
rigidbody.useGravity = false;
}


transform.Translate(Vector3.forward *Speed* Time.deltaTime);

if(Input.GetAxis("Accelerate"))
{
Speed = Speed + Acceleration;
}

if (Input.GetAxis("Decelerate"))
{
Speed = Speed - Deceleration;
}

if (Input.GetAxis("Menu"))
{
Application.LoadLevel("Pause Menu");
}

transform.Rotate(0,Input.GetAxis("Turn") * YawSpeed * Time.deltaTime,0);

transform.Rotate(0,0,Input.GetAxis("Yaw") * TurnSpeed * Time.deltaTime);

transform.Rotate(Input.GetAxis("Elevate") * ElevateSpeed * 2 *Time.deltaTime,0,0);
}

Any ideas?