Validate CheckBox selection from multiple CheckBoxes using jQuery

You can try below code:

<asp:checkboxlist id="CheckBoxList1" runat="server">
<asp:ListItem>a</asp:ListItem>
<asp:ListItem>b</asp:ListItem>
<asp:ListItem>c</asp:ListItem>
</asp:checkboxlist>
<asp:button id="Button1" runat="server" text="Button" onclientclick="return ValidateCheckBoxList()" />

<script>
    function ValidateCheckBoxList() {
        var listItems = document.getElementById("CheckBoxList1").getElementsByTagName("input");
        var itemcount = listItems.length;
        var iCount = 0;
        var isItemSelected = false;
        for (iCount = 0; iCount < itemcount; iCount++) {
            if (listItems[iCount].checked) {
                isItemSelected = true;
                break;
            }
        }
        if (!isItemSelected) {
            alert("Please select an Item.");
        }
        else {
            return true;
        }
        return false;
    }
</script>

Hope it helps you.