help needed please problem with products and shopping cart

hi there sorry to be a pain but i am in need of a bit of help as i am having problems here https://www.buynewandused.co.uk when i try and view and add products to the cart.
i have run debug utility and heres the result

[24-Sep-2008 15:00:50] PHP Fatal error: main(): Failed opening required '' (include_path='.:/usr/share/pear') in /home/.sites/112/site7/web/includes/templates/template_default/templates/tpl_page_not_found_default.php on line 23

but i am confused on how to remidy this any help would be appreciated
thanks in advance
[email protected]

Have you recently added or switched to new template?
Have you recently added any new mods/additions/sideboxes?
Does error still appear when using CLASSIC template?

hi first of all thanks for your time
i have not changed template i added shopping cart to define_pages editor from the zencart free addons and it did not work so i deleted only the files i uploaded to remove it and this has been happening since then.
i have tried different templates and it is still happening
do you have any ideas on how to remidy this?

thanks again from [email protected]

Think you may have deleted more than you should have..
Visited site and there are also language files missing..

i added shopping cart to define_pages editor from the zencart free addons and it did not work so i deleted only the files i uploaded to remove it and this has been happening since then.
Did you backup your site and database before adding
addition above?
If it is happening on all templates.. especially Classic, then
there is definitely file/s missing.

ran my backup and still happening tried allsorts pulling my hair out now lol
any more ideas?
thanks again