textbox's textchanged event not fired

<asp:Panel ID="panelfrmtodt" runat="server">
<table>
<tr>
<td style="text-align: left; vertical-align: top; width: 69px; height: 23px;" align="left">
<asp:Label ID="lblfrmdt" runat="server" Text="From Date" ForeColor="#175DAA" Font-Names="Verdana"
Font-Size="8pt"></asp:Label>
</td>
<td style="text-align: left; vertical-align: top; height: 23px;" align="left">
<asp:TextBox ID="txtfrmdt" runat="server" Font-Names="Verdana" Font-Size="8pt" ForeColor="#175DAA"></asp:TextBox>
</td>
<td style="text-align: left; vertical-align: top; height: 23px;" align="left">
<asp:Image ID="imgfrmdt" runat="server" ImageUrl="~/images/Calendar_scheduleHS.png"
ForeColor="Desktop"></asp:Image>
</td>
<td style="text-align: left; vertical-align: top; height: 23px;" align="left">
<asp:Label ID="lbltodt" runat="server" Text="To Date" ForeColor="#175DAA" Font-Names="Verdana"
Font-Size="8pt"></asp:Label>
</td>
<td style="text-align: left; vertical-align: top; height: 23px;" align="left">
<asp:TextBox ID="txttodt" runat="server" Font-Names="Verdana" Font-Size="8pt" ForeColor="#175DAA"
OnTextChanged="txttodt_TextChanged" AutoPostBack="True" CausesValidation="True"></asp:TextBox>
</td>
<td style="text-align: left; vertical-align: top; height: 23px;" align="left">
<asp:Image ID="imgtodt" runat="server" ImageUrl="~/images/Calendar_scheduleHS.png"
ForeColor="Desktop"></asp:Image>
</td>
</tr>
</table>
</asp:Panel>


cs file
protected void txttodt_TextChanged(object sender, EventArgs e)
{
showgrid();
}