Could not load file or assembly ‘AjaxControlToolkit’ or one of its dependencies. Access is denied.

I have solved my problem deleting my temporary files in both .net frameworks folders

C:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files

For 64bit systems, also check the same under the Framework*64* folders i.e.

C:\Windows\Microsoft.NET\Framework64\v2.0.50727\Temporary ASP.NET Files
C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files

Delete Multiple Cookies in Sign out button click

            HttpCookie aCookie;
            string cookieName;
            int limit = Request.Cookies.Count;
            for (int i = 0; i < limit; i++)
            {
                cookieName = Request.Cookies[i].Name;
                aCookie = new HttpCookie(cookieName);
                aCookie.Expires = DateTime.Now.AddDays(-1);
                Response.Cookies.Add(aCookie);
            }

       
            Response.Redirect("/myhomepage.aspx");

Find Mastepage Control in webpage:

Page.Master.FindControl("idname").Visible = false;