Hi Swarup,
Some changes make your code, it is fine work.
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<script type="text/javascript">
function alpha(e)
{
var k;
document.all ? k = e.keyCode : k = e.which;
return ((k > 64 && k < 91) || (k > 96 && k < 123) || k == 8 || k==32 || k==13);
}
</script>
<asp:TextBox ID="txtpartyhallname" runat="server" Width="200px" CssClass="WaterMarkedTextBox" onkeydown="return alpha(event)" ></asp:TextBox>
<asp:ImageButton ID="ImageButton2" runat="server"
onclick="ImageButton2_Click" />
</asp:Content>