Asp.net controls are not accessible in code behind

I have a project that was previously built by someone else. When I try to add a new control to the page, I can't access it in the code behind. When I try to access it there's an error message that the Control is not defined in the current context.

<%@ Page Language="C#" MasterPageFile="~/HRMS.Master" AutoEventWireup="true"     CodeBehind="manageEmployee.aspx.cs" Inherits="HRMS.manageEmployee" Title="Untitled Page" %>

<%@ Register Assembly="eWorld.UI" Namespace="eWorld.UI" TagPrefix="ew" %>
<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="cc1" %>


<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
    <script type="text/javascript">
    document.getElementById("employee").setAttribute("class", "selected");
    </script>
<!--OPEN CONTENT DIV-->

     <asp:ScriptManager ID="ScriptManager1" runat="server">
     </asp:ScriptManager>
    <!-- Only this area is updated on PostBack -->

<div class="manage_employ">

    <ul>
        <li style="margin-top:10px; margin-bottom:15px"><input type="image" src="images/img_manageEmployee01.png"/></li>
    </ul>

    <ul>
        <li>First name</li>
        <li><asp:TextBox runat="server" class="text_input" id="txtFirstName"/></li>
        <asp:RequiredFieldValidator ID="validatorFirstName" runat="server" ErrorMessage="*" ControlToValidate="txtFirstName"></asp:RequiredFieldValidator>
        <li>Last name</li>
        <li><asp:TextBox runat="server" class="text_input" id="txtLastName"/></li>
        <asp:RequiredFieldValidator ID="validatorLastname" runat="server" ErrorMessage="*" ControlToValidate="txtLastName"></asp:RequiredFieldValidator>
    </ul>

    &l