WHRER DID you add the event in HTML part. try add something like this
<asp:textbox id="TextBox1" runat="server" height="20px" width="150px" autocomplete="off" validationgroup="TxtBox1" ontextchange="TextBox1_TextChanged" autopostback="True" font-bold="True" font-size="Small" xmlns:asp="#unknown">
</asp:textbox>