Based on your previous information:

if (buttonWasClicked)
{
    progressBar.progess = 0;
}

Based on your update with the link, I am guessing that you want it to be like this:

var barDisplay : float = 0;
var pos : Vector2 = new Vector2(20,40);
var size : Vector2 = new Vector2(60,20);
var progressBarEmpty : Texture2D;
var progressBarFull : Texture2D;

function OnGUI()
{

    // draw the background:
    GUI.BeginGroup (new Rect (pos.x, pos.y, size.x, size.y));

        GUI.Box (Rect (0,0, size.x, size.y),progressBarEmpty);

        // draw the filled-in part:
        GUI.BeginGroup (new Rect (0, 0, size.x * barDisplay, size.y));
            GUI.Box (Rect (0,0, size.x, size.y),progressBarFull);
        GUI.EndGroup ();

    GUI.EndGroup ();

} 

function Update()
{
    barDisplay += Time.deltaTime * 0.05;
    barDisplay = Mathf.Clamp01(barDisplay);
}

function ResetBar()
{
    barDisplay = 0.0f;
}

You can then control the bar by setting enabled = true when you want the bar to tick up, and call ResetBar() when you want it to reset (or just modify barDisplay, it runs between 0 and 1).