Hi aspvijay04,

Check this example. Now please take its reference and correct your code.

Database

I have used Employees table from Northwind database.

Install Microsoft Northwind and Pubs Sample databases in SQL Server Management Studio

HTML

<div>
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
    <script type="text/javascript" src="http://cdn.jsdelivr.net/jquery.simpletip/1.3.1/jquery.simpletip-1.3.1.min.js"></script>
    <style type="text/css">
        .tooltip
        {
            position: absolute;
            top: 0;
            left: 0;
            z-index: 3;
            display: none;
            background-color: #FB66AA;
            color: White;
            padding: 5px;
            font-size: 10pt;
            font-family: Arial;
        }
        td
        {
            cursor: pointer;
        }            
    </style>
    <script type="text/javascript">
        $(function () {
            var row;
            $('[id*=GridView1] tr:has(td)').mouseover(function () {
                var employeeId = $(this).find("td:first").html();
                row = $(this);
                $.ajax({
                    type: "POST",
                    url: '<%=ResolveUrl("Default.aspx/GetEmployeeAddress") %>',
                    data: '{employeeId: "' + employeeId + '" }',
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                    success: function (response) {
                        row.find("td").each(function () {
                            $(this).simpletip({
                                content: response.d
                            });
                        });
                    },
                    failure: function (response) {
                        alert(response.d);
                    }
                });
            });
        });
         
    </script>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField DataField="EmployeeId" HeaderText="EmployeeId" ItemStyle-Width="30" />
            <asp:BoundField DataField="FirstLast" HeaderText="Name" ItemStyle-Width="180" />
            <asp:BoundField DataField="Country" HeaderText="Country" ItemStyle-Width="150" />
        </Columns>
    </asp:GridView>
</div>

Namespaces

using System.Configuration;
using System.Data.SqlClient;
using System.Data;

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.PopulateEmployees();
    }
}

[System.Web.Services.WebMethod]
public static string GetEmployeeAddress(string employeeId)
{
    string address = string.Empty;
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection conn = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT Address + ',' + City + ',' + Country 'Address' FROM Employees WHERE EmployeeId = @EmployeeId", conn))
        {
            cmd.Parameters.AddWithValue("@EmployeeId", employeeId);
            conn.Open();
            address = cmd.ExecuteScalar().ToString();
            conn.Close();
            return address;
        }
    }
}

private void PopulateEmployees()
{
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection conn = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT EmployeeId, FirstName + ' ' + LastName FirstLast, Country FROM Employees", conn))
        {
            using (SqlDataAdapter da = new SqlDataAdapter(cmd))
            {
                DataSet ds = new DataSet();
                da.Fill(ds);
                GridView1.DataSource = ds;
                GridView1.DataBind();
            }
        }
    }
}

Screenshot