<asp:Content ID="Content2" ContentPlaceHolderID="cphContent" runat="Server">
    <script type="text/javascript">
        function DispValue(sender, e) {
            var id = e.get_value();
            document.getElementById("<%=PatientRefferalId.ClientID%>").value=id; //getting error here
        }
    </script>

    <div align="left" style="float: left; margin-left: 5px;">
        <asp:GridView ID="gvPatient" runat="server" AutoGenerateColumns="false" EnableViewState="true">
            <Columns>
                <asp:TemplateField HeaderStyle-Font-Bold="true" HeaderStyle-Font-Size="12px" HeaderStyle-Height="20px">
                    <HeaderTemplate>&nbsp;Patient Name&nbsp;</HeaderTemplate>
                    <ItemTemplate>
                        <asp:HiddenField ID="PatientRefferalId" runat="server" Value="0" />
                        <PUC:PatientUserControl ID="pucPatient1" runat="server" OnClientSelect="DispValue" PTStatusShow="0"/>
                    </ItemTemplate>
                </asp:TemplateField>
            </Columns>
        </asp:GridView>
    </div>
</asp:Content>

protected void Page_Load(object sender, EventArgs e) {
    try {
        if (!IsPostBack) {
            dt = new DataTable();
            dt.Columns.Add("col1");
            dt.Columns.Add("col2");
            dt = AddRow(dt);
            gvPatient.DataSource = dt;
            gvPatient.DataBind();
        }
    } catch (Exception ex) {

    }
}

private DataTable AddRow(DataTable dt) {
    for (int i = 0; i < 5; i++) {
        DataRow dr = dt.NewRow();
        dr[0] = "";
        dr[1] = ""; dt.Rows.Add(dr);
    }
    return dt;
}

protected void GridPatient_DataBound(object sender, EventArgs e) {
    try {
        foreach (GridViewRow item in gvPatient.Rows) {
            HiddenField hfReferralId = (HiddenField)item.FindControl("PatientRefferalId");
            Response.write(hfReferralId.Value);
        }
    } catch (Exception ex) {

    }
}

document.getElementById("<%=PatientRefferalId.ClientID%>")

var gv = document.getElementById("<%=gvPatient.ClientID%>");
var Rows = gv.getElementsByTagName("tr"); // Get all the rows from your gridview (rendered as html table).
// you can loop through the rows or if you know the row index, you can do:
alert(Rows[2].childNodes[0].children[0].value); // Show you the first control (the hiddenfield) of the first cell of the row #2.

var c = document.getElementsByTagName("table");
    for (var i = 0; i < c.length; i++) {
        if (c[i].id.indexOf("GridView1") > -1) {
            var hidd = c[i].getElementsByTagName("input");
            for (var j = 0; j < hidd.length; j++) {
                if (hidd[j].type == "hidden")
                    alert(hidd[j].id);
            }
        }
    }

<asp:Content ID="Content2" ContentPlaceHolderID="cphContent" runat="Server">
        <script type="text/javascript">
            function DispValue(sender, e) {
    var tGrid = document.getElementById('<%= gvPatient.ClientID%>');
       var parentRow = $(btnShow).closest("tr");
        var hiddenField=parentRow.find('input[id$=PatientRefferalId]');
    alert(hiddenField);
    return false;
            }
        </script>

        <div align="left" style="float: left; margin-left: 5px;">
            <asp:GridView ID="gvPatient" runat="server" AutoGenerateColumns="false" EnableViewState="true">
                <Columns>
                    <asp:TemplateField HeaderStyle-Font-Bold="true" HeaderStyle-Font-Size="12px" HeaderStyle-Height="20px">
                        <HeaderTemplate>&nbsp;Patient Name&nbsp;</HeaderTemplate>
                        <ItemTemplate>
                            <asp:HiddenField ID="PatientRefferalId" runat="server" Value="0" />

  <asp:LinkButton ID="lnkPopUp" runat="server" Style="font-size: 16px;" OnClientClick="return DispValue(this)"  Text="PopUp"
                                                       ></asp:LinkButton>

                        </ItemTemplate>
                    </asp:TemplateField>
                </Columns>
            </asp:GridView>
        </div>
    </asp:Content>