Hi indradeo,

In CLOSING_DATE set DBNull.Value and Status column set Open as parameter value.

Refer below updated code.

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        ViewState["Filter"] = "ALL";
        BindGrid();
    }
}

private void BindGrid()
{
    DataTable dt = new DataTable();
    String strConnString = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
    SqlConnection con = new SqlConnection(strConnString);
    SqlDataAdapter sda = new SqlDataAdapter();
    SqlCommand cmd = new SqlCommand("spx_GetOCMS");
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("@Filter", ViewState["Filter"].ToString());
    cmd.Connection = con;
    sda.SelectCommand = cmd;
    sda.Fill(dt);
    GridView1.DataSource = dt;
    GridView1.DataBind();
    DropDownList ddlCountry = (DropDownList)GridView1.HeaderRow.FindControl("ddlCountry");
    this.BindCountryList(ddlCountry);
}

protected void CountryChanged(object sender, EventArgs e)
{
    DropDownList ddlCountry = (DropDownList)sender;
    ViewState["Filter"] = ddlCountry.SelectedValue;
    this.BindGrid();
}

private void BindCountryList(DropDownList ddlCountry)
{
    String strConnString = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
    SqlConnection con = new SqlConnection(strConnString);
    SqlDataAdapter sda = new SqlDataAdapter();
    SqlCommand cmd = new SqlCommand("select distinct STATUS_NAME from complaint_status");
    cmd.Connection = con;
    con.Open();
    ddlCountry.DataSource = cmd.ExecuteReader();
    ddlCountry.DataTextField = "STATUS_NAME";
    ddlCountry.DataValueField = "STATUS_NAME";
    ddlCountry.DataBind();
    con.Close();
    ddlCountry.Items.FindByValue(ViewState["Filter"].ToString()).Selected = true;
}

protected void OnPaging(object sender, GridViewPageEventArgs e)
{
    GridView1.PageIndex = e.NewPageIndex;
    this.BindGrid();
}

protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
{
    if (e.CommandName == "EditButton")
    {
        int index = Convert.ToInt32(e.CommandArgument);
        GridViewRow row = GridView1.Rows[index];
        Response.Redirect("~/WebForm3.aspx?COMP_ID=" + row.Cells[0].Text);
    }
}

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        if (e.Row.Cells[7].Text == "Y")
        {
            (e.Row.FindControl("LinkButton1") as LinkButton).Visible = false;
        }
    }
}

protected void LinkButton1_Click(object sender, EventArgs e)
{
    GridViewRow row = (sender as LinkButton).NamingContainer as GridViewRow;
    int compId = Convert.ToInt32(row.Cells[0].Text.Trim());
    string conString = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
    string query = "UPDATE comp_box SET STATUS_ID = @Status, CLOSING_DATE=@CLOSING_DATE WHERE COMP_ID = @CompId";
    using (SqlConnection con = new SqlConnection(conString))
    {
        SqlCommand cmd = new SqlCommand();
        cmd.CommandText = query;
        cmd.CommandType = CommandType.Text;
        cmd.Parameters.AddWithValue("@CompId", compId);
        cmd.Parameters.AddWithValue("@Status", "Open");
        cmd.Parameters.AddWithValue("@CLOSING_DATE", DBNull.Value);
        cmd.Connection = con;
        con.Open();
        cmd.ExecuteNonQuery();
        con.Close();
    }
    this.BindGrid();
}