Create a class and add these two methods to it :
 
// for postback events
public static void ShowMessageBox(string _message)
    {
        Page page = HttpContext.Current.Handler as Page;
        page.RegisterStartupScript("key2", "<script>alert('" + _message +  "');</script>");
    }
 
// for async postback events
 public static void ShowMessageBoxForAsync(string _message)
    {
        Page page = HttpContext.Current.Handler as Page;
        ScriptManager.RegisterClientScriptBlock(page,page.GetType(), "key2", "alert('" + _message + "');", true);
    }
 
// Then you can call this in, let's say , button_click event handler like this ( i assume that your class name is Utilities without any namespace definition) :
protected void myButton_Click(object sender, EventArgs e)
{
Utilities.ShowMessageBox("you entered a wrong password");
}