Hi Bhavesh23,

Please refer below sample.

HTML

<asp:DropDownList runat="server" ID="dropblockselection" AutoPostBack="true"
    OnSelectedIndexChanged="ddlDetails_SelectedIndexChanged">
    <asp:ListItem Text="Select" />
    <asp:ListItem Text="1 To 7" Value="1" />
    <asp:ListItem Text="8 To 14" Value="2" />
    <asp:ListItem Text="P1/P2" Value="3" />
    <asp:ListItem Text="Meetingroom" Value="4" />
</asp:DropDownList>
<hr />
<asp:CheckBoxList runat="server" ID="chkblockselection" RepeatColumns="4" 
    RepeatDirection="Horizontal" Height="157px" Width="289px" CssClass="auto-style1" 
    BorderStyle="Solid" Font-Bold="True" AutoPostBack="True" Enabled="False">
    <asp:ListItem Text="0" Value="0" />
    <asp:ListItem Text="1" Value="1" />
    <asp:ListItem Text="2" Value="2" />
    <asp:ListItem Text="3" Value="3" />
    <asp:ListItem Text="4" Value="4" />
    <asp:ListItem Text="5" Value="5" />
    <asp:ListItem Text="6" Value="6" />
    <asp:ListItem Text="7" Value="7" />
    <asp:ListItem Text="8" Value="8" />
    <asp:ListItem Text="9" Value="9" />
    <asp:ListItem Text="10" Value="10" />
    <asp:ListItem Text="11" Value="11" />
    <asp:ListItem Text="12" Value="12" />
    <asp:ListItem Text="13" Value="13" />
    <asp:ListItem Text="14" Value="14" />
    <asp:ListItem Text="15" Value="15" />
    <asp:ListItem Text="16" Value="16" />
</asp:CheckBoxList>

Code

C#

protected void ddlDetails_SelectedIndexChanged(object sender, EventArgs e)
{
    foreach (ListItem item in chkblockselection.Items)
    {
        item.Selected =false;
    }
    if (dropblockselection.SelectedItem.Text == "1 To 7")
    {
        chkblockselection.Enabled = true;
        int start = Convert.ToInt16(dropblockselection.SelectedItem.Text.ToLower().Split(new string[] { "to" }, StringSplitOptions.None)[0].Trim()) - 1;
        int end = Convert.ToInt16(dropblockselection.SelectedItem.Text.ToLower().Split(new string[] { "to" }, StringSplitOptions.None)[1].Trim());
        foreach (ListItem item in chkblockselection.Items)
        {
            item.Enabled = Convert.ToInt16(item.Value) >= start && Convert.ToInt16(item.Value) < end ? true : false;
        }
    }
    if (dropblockselection.SelectedItem.Text == "8 To 14")
    {
        chkblockselection.Enabled = true;
        int start = Convert.ToInt16(dropblockselection.SelectedItem.Text.ToLower().Split(new string[] { "to" }, StringSplitOptions.None)[0].Trim()) - 1;
        int end = Convert.ToInt16(dropblockselection.SelectedItem.Text.ToLower().Split(new string[] { "to" }, StringSplitOptions.None)[1].Trim());
        foreach (ListItem item in chkblockselection.Items)
        {
            item.Enabled = Convert.ToInt16(item.Value) >= start && Convert.ToInt16(item.Value) < end ? true : false;
        }
    }
    if (dropblockselection.SelectedItem.Text == "P1/P2")
    {
        chkblockselection.Enabled = true;
        int start = 14;
        int end = 15;
        foreach (ListItem item in chkblockselection.Items)
        {
            item.Enabled = Convert.ToInt16(item.Value) >= start && Convert.ToInt16(item.Value) <= end ? true : false;
        }
        chkblockselection.Items.FindByValue("16").Enabled = false;
    }
    if (dropblockselection.SelectedItem.Text == "Meetingroom")
    {
        chkblockselection.Enabled = true;
        int start = 0;
        int end = 16;
        foreach (ListItem item in chkblockselection.Items)
        {
            item.Enabled = Convert.ToInt16(item.Value) >= start && Convert.ToInt16(item.Value) < end ? false : true;
        }
        chkblockselection.Items.FindByValue("16").Enabled = true;
    }
}

VB.Net

Protected Sub ddlDetails_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    For Each item As ListItem In chkblockselection.Items
        item.Selected = False
    Next

    If dropblockselection.SelectedItem.Text = "1 To 7" Then
        chkblockselection.Enabled = True
        Dim start As Integer = Convert.ToInt16(dropblockselection.SelectedItem.Text.ToLower().Split(New String() {"to"}, StringSplitOptions.None)(0).Trim()) - 1
        Dim [end] As Integer = Convert.ToInt16(dropblockselection.SelectedItem.Text.ToLower().Split(New String() {"to"}, StringSplitOptions.None)(1).Trim())

        For Each item As ListItem In chkblockselection.Items
            item.Enabled = If(Convert.ToInt16(item.Value) >= start AndAlso Convert.ToInt16(item.Value) < [end], True, False)
        Next
    End If

    If dropblockselection.SelectedItem.Text = "8 To 14" Then
        chkblockselection.Enabled = True
        Dim start As Integer = Convert.ToInt16(dropblockselection.SelectedItem.Text.ToLower().Split(New String() {"to"}, StringSplitOptions.None)(0).Trim()) - 1
        Dim [end] As Integer = Convert.ToInt16(dropblockselection.SelectedItem.Text.ToLower().Split(New String() {"to"}, StringSplitOptions.None)(1).Trim())

        For Each item As ListItem In chkblockselection.Items
            item.Enabled = If(Convert.ToInt16(item.Value) >= start AndAlso Convert.ToInt16(item.Value) < [end], True, False)
        Next
    End If

    If dropblockselection.SelectedItem.Text = "P1/P2" Then
        chkblockselection.Enabled = True
        Dim start As Integer = 14
        Dim [end] As Integer = 15

        For Each item As ListItem In chkblockselection.Items
            item.Enabled = If(Convert.ToInt16(item.Value) >= start AndAlso Convert.ToInt16(item.Value) <= [end], True, False)
        Next

        chkblockselection.Items.FindByValue("16").Enabled = False
    End If

    If dropblockselection.SelectedItem.Text = "Meetingroom" Then
        chkblockselection.Enabled = True
        Dim start As Integer = 0
        Dim [end] As Integer = 16

        For Each item As ListItem In chkblockselection.Items
            item.Enabled = If(Convert.ToInt16(item.Value) >= start AndAlso Convert.ToInt16(item.Value) < [end], False, True)
        Next

        chkblockselection.Items.FindByValue("16").Enabled = True
    End If
End Sub

Screenshot