Finally I got the solution :-
protected void BtnSave_Click(object sender, EventArgs e)
{
using (SampleDataContext dbContext = new SampleDataContext())
{
System.Nullable<char> outpt = null;
long id = Convert.ToInt64(Session["USERID"]);
dbContext.passwrdchange(id, old.Text.ToString().Trim(), newpass.Text.ToString().Trim(), ref outpt);
if(outpt.ToString()=="T")
{
ClientScript.RegisterStartupScript(this.GetType(), "SuccessMsg", "<script>alert('Password Changed Successfully')</script>");
}
else
{
ClientScript.RegisterStartupScript(this.GetType(), "ErrorMsg", "<script>alert('Password Not Changed')</script>");
}
}
}