The next or previous button are not working after searching the item.

The next or previous button is not working after searching the item. Before searching, the next or previous button was working but after searching by id and when I click next, it will back to the first id number instead of continuous after searching number. Here is the code below

<div>
       <table><tr><td><asp:TextBox ID="txtsearch" runat="server" /></td>
           <td> <asp:Button ID="Button1" OnClick="Button1_Click" runat="server" Text="Button" /></td></tr></table> 
        <br />
        <table>
            <tr><td><asp:TextBox ID="txtid" runat="server"></asp:TextBox></td></tr>
            <tr><td><asp:TextBox ID="txtfirstname" runat="server"></asp:TextBox></td></tr>
            <tr><td> <asp:TextBox ID="txtlastname" runat="server"></asp:TextBox></td><td></td></tr>
        </table>
        <div>
            <asp:Button ID="btnprevious" runat="server" OnClick="btnprevious_Click" Text="Previous" /><asp:Button ID="btnnext" OnClick="btnnext_Click" runat="server" Text="Next" /> </div>
    </div>

public partial class WebForm1 : System.Web.UI.Page
    {
        DataTable sessionDataTable = new DataTable();
        int pos = 0;
        DataTable dt = new DataTable();
        int index = 0;
        int last = 0;
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                getbind();
            } 
        }
        private int DataIndex
        {
            get
            {
                object viewState = this.ViewState["DataIndex"];

                return (viewState == null) ? 0 : (int)viewState;
            }
            set
            {
                this.ViewState["DataIndex"] = value;
            }
        }
        SqlDataAdapter da;
        private void getbind()
        {
           object sessionObject = this.Session["StatesDataTable"];
           
                string querystr = ConfigurationManager.ConnectionStrings["NSRADConnectionString"].ConnectionString;
                SqlConnection con = new SqlConnection(querystr);
          
                da = new SqlDataAdapter("SELECT ID,FirstName,LastName from RadTable", con);
                da.Fill(sessionDataTable);

                this.Session["StatesDataTable"] = sessionDataTable;
                sessionObject = sessionDataTable;
          
            DataTable dataTable = (DataTable)sessionObject;
          
            if (this.DataIndex < 0) // Roll to the last item
                this.DataIndex = (dataTable.Rows.Count - 1);
            else if (this.DataIndex > (dataTable.Rows.Count - 1))
                this.DataIndex = 0; // Roll to the first item

            if (dataTable.Rows.Count <= 0)
                return;
            txtid.Text = dataTable.Rows[DataIndex]["ID"].ToString();
            txtfirstname.Text = dataTable.Rows[DataIndex]["FirstName"].ToString();
            txtlastname.Text = dataTable.Rows[DataIndex]["LastName"].ToString();
        }
        protected void Button1_Click(object sender, EventArgs e)
        {
            search();
        }
        private void search()
        {
            object sessionObject = this.Session["StatesDataTable"];
            string querystr = ConfigurationManager.ConnectionStrings["NSRADConnectionString"].ConnectionString;
            SqlConnection con = new SqlConnection(querystr);

            SqlDataAdapter da = new SqlDataAdapter("SELECT ID,FirstName,LastName from RadTable Where ID = '" + txtsearch.Text + "'", con);
            DataTable sessionDataTable = new DataTable();
            da.Fill(sessionDataTable);
            this.Session["StatesDataTable"] = sessionDataTable;
            sessionObject = sessionDataTable;

            DataTable dataTable = (DataTable)sessionObject;
            // Make sure that the index is in range...  
            if (this.DataIndex < 0) // Roll to the last item
                this.DataIndex = (dataTable.Rows.Count - 1);
            else if (this.DataIndex > (dataTable.Rows.Count - 1))
            {
               this.DataIndex = 0; // Roll to the first item
                //this.DataIndex = Convert.ToInt32(txtid.Text);
            }

            if (dataTable.Rows.Count <= 0)
                return;

            txtid.Text = dataTable.Rows[DataIndex]["ID"].ToString();
            txtfirstname.Text = dataTable.Rows[DataIndex]["FirstName"].ToString();
            txtlastname.Text = dataTable.Rows[DataIndex]["LastName"].ToString();
        }
        private void listban(int index)
        {
            txtid.Text = dt.Rows[index]["ID"].ToString();
            txtfirstname.Text = dt.Rows[index]["FirstName"].ToString();
            txtlastname.Text = dt.Rows[index]["LastName"].ToString();
        }
        protected void btnnext_Click(object sender, EventArgs e)
        {
            if (txtsearch.Text != "")
            {
                this.DataIndex++;
                search();
            }
            else
            {
                this.DataIndex++;
                getbind();
            }
           
        }

        protected void btnprevious_Click(object sender, EventArgs e)
        {
            this.DataIndex--;

            getbind();
        }