Checkbox problem

I have a  datagrid with an HTML checkbox in a template column.  I am having problems extracting the values of the checked items. I have tried using javascript, but I keep getting errors informing me that this checkbox is null or not an object.

Can anyone help? Thanks.

Here is a snippet of code.

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   <asp:datagrid id="dgExistingList" runat="server" DataKeyField="PROJECT_NUMBER" AllowSorting="True" AutoGenerateColumns="False" Width="730px">
              <AlternatingItemStyle Font-Size="12px" Font-Names="Arial" BackColor="#E2F6F1"></AlternatingItemStyle>
              <ItemStyle Font-Size="12px" Font-Names="Arial"></ItemStyle>
              <HeaderStyle Font-Size="12px" Font-Names="Arial" Font-Bold="True" BackColor="#DDDDDD"></HeaderStyle>
              <Columns>
               <asp:TemplateColumn Visible="False">
                 <HeaderStyle Width="10px"></HeaderStyle>
                <HeaderTemplate>
                 &nbsp;
                </HeaderTemplate>
                <ItemTemplate>
                 <INPUT type="checkbox" id="chkProjectNumbers" runat="server" name="chkProjectNumbers" value='<%# DataBinder.Eval(Container, "DataItem.PROJECT_NUMBER") %>'>
                 </asp:HyperLink>
                </ItemTemplate>

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