Hi Richa,

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

For sum i have used DataTable Compute function. You can refer below article for more details.

Calculate Sum (Total) of DataTable Columns using C# and VB.Net

HTML

<asp:DropDownList runat="server" ID="ddlYears" AutoPostBack="true" OnSelectedIndexChanged="YearChanged">
    <asp:ListItem Text="2018" Value="2018" />
    <asp:ListItem Text="2019" Value="2019" />
    <asp:ListItem Text="2020" Value="2020" />
    <asp:ListItem Text="2021" Value="2021" />
    <asp:ListItem Text="2022" Value="2022" />
</asp:DropDownList>
<br />
Sum of values for all the years for Region = PM à <b>
    <asp:Label ID="lblPM" runat="server" /></b>
<br />
Sum of values for all the years for Region = MSw à <b>
    <asp:Label ID="lblMSw" runat="server" /></b>
<br />
Sum of values for all the years for Region = MS à <b>
    <asp:Label ID="lblMS" runat="server" /></b>
<br />
<asp:GridView runat="server" ID="gvDetails" />

Namespace

C#

using System.Data;

VB.Net

Imports System.Data

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        DataTable dt = GetData();
        gvDetails.DataSource = dt;
        gvDetails.DataBind();
        lblMSw.Text = dt.Compute("SUM(value)", "region = 'MSw'").ToString();
        lblPM.Text = dt.Compute("SUM(value)", "region = 'PM'").ToString();
        lblMS.Text = dt.Compute("SUM(value)", "region = 'MS'").ToString();
    }
}

protected void YearChanged(object sender, EventArgs e)
{
    DataTable dt = GetData();
    DataRow[] dr = dt.Select("year>=" + ddlYears.SelectedValue);
    DataTable dtFilter = dr.Length > 0 ? dr.CopyToDataTable() : dt.Clone();
    gvDetails.DataSource = dtFilter;
    gvDetails.DataBind();
    lblMSw.Text = dt.Compute("SUM(value)", "year>= " + ddlYears.SelectedValue + " AND region = 'MSw'").ToString();
    lblPM.Text = dt.Compute("SUM(value)", "year>= " + ddlYears.SelectedValue + " AND region = 'PM'").ToString();
    lblMS.Text = dt.Compute("SUM(value)", "year>= " + ddlYears.SelectedValue + " AND region = 'MS'").ToString();
}

private DataTable GetData()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[3] { 
                        new DataColumn("year", typeof(int)),
                        new DataColumn("value", typeof(decimal)),
                        new DataColumn("region",typeof(string)) });
    dt.Rows.Add(2018, 999, "MSw");
    dt.Rows.Add(2018, 11770.9, "PM");
    dt.Rows.Add(2018, 37.6, "MS");
    dt.Rows.Add(2019, 550.6, "PM");
    dt.Rows.Add(2019, 1888.3, "MSw");
    dt.Rows.Add(2019, 827.3, "MS");
    dt.Rows.Add(2020, 129144.9, "MSw");
    dt.Rows.Add(2020, 2466.3, "PM");
    dt.Rows.Add(2020, 1692.6, "MS");
    dt.Rows.Add(2021, 515.4, "PM");
    dt.Rows.Add(2021, 2.5, "MS");
    dt.Rows.Add(2021, 1550.3, "MSw");
    dt.Rows.Add(2022, 343, "MS");
    dt.Rows.Add(2022, 12, "MSw");
    dt.Rows.Add(2022, 45.6, "PM");
    return dt;
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not IsPostBack Then
        Dim dt As DataTable = GetData()
        gvDetails.DataSource = dt
        gvDetails.DataBind()
        lblMSw.Text = dt.Compute("SUM(value)", "region = 'MSw'").ToString()
        lblPM.Text = dt.Compute("SUM(value)", "region = 'PM'").ToString()
        lblMS.Text = dt.Compute("SUM(value)", "region = 'MS'").ToString()
    End If
End Sub

Protected Sub YearChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim dt As DataTable = GetData()
    Dim dr As DataRow() = dt.[Select]("year>=" & ddlYears.SelectedValue)
    Dim dtFilter As DataTable = If(dr.Length > 0, dr.CopyToDataTable(), dt.Clone())
    gvDetails.DataSource = dtFilter
    gvDetails.DataBind()
    lblMSw.Text = dt.Compute("SUM(value)", "year>= " & ddlYears.SelectedValue & " AND region = 'MSw'").ToString()
    lblPM.Text = dt.Compute("SUM(value)", "year>= " & ddlYears.SelectedValue & " AND region = 'PM'").ToString()
    lblMS.Text = dt.Compute("SUM(value)", "year>= " & ddlYears.SelectedValue & " AND region = 'MS'").ToString()
End Sub

Private Function GetData() As DataTable
    Dim dt As DataTable = New DataTable()
    dt.Columns.AddRange(New DataColumn(2) {New DataColumn("year", GetType(Integer)),
                                           New DataColumn("value", GetType(Decimal)),
                                           New DataColumn("region", GetType(String))})
    dt.Rows.Add(2018, 999, "MSw")
    dt.Rows.Add(2018, 11770.9, "PM")
    dt.Rows.Add(2018, 37.6, "MS")
    dt.Rows.Add(2019, 550.6, "PM")
    dt.Rows.Add(2019, 1888.3, "MSw")
    dt.Rows.Add(2019, 827.3, "MS")
    dt.Rows.Add(2020, 129144.9, "MSw")
    dt.Rows.Add(2020, 2466.3, "PM")
    dt.Rows.Add(2020, 1692.6, "MS")
    dt.Rows.Add(2021, 515.4, "PM")
    dt.Rows.Add(2021, 2.5, "MS")
    dt.Rows.Add(2021, 1550.3, "MSw")
    dt.Rows.Add(2022, 343, "MS")
    dt.Rows.Add(2022, 12, "MSw")
    dt.Rows.Add(2022, 45.6, "PM")
    Return dt
End Function

Screenshot