please check your code once again.. below is working
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><asp:RequiredFieldValidator runat="server" ID="rfvtxt1" ControlToValidate="TextBox1" ErrorMessage="Required"></asp:RequiredFieldValidator>
<asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
<asp:Label ID="Label1" runat="server" Text=""></asp:Label>
</ContentTemplate>
</asp:UpdatePanel>
and code behind
protected void Button1_Click(object sender, EventArgs e)
{
Label1.Text = "the value is:" + TextBox1.Text;
}