fatal error

we upgraded our opencart a while back and its been working on and off with a few issues - we are using ver 2.3.0.2 now.

our site has gone down and we are getting this error : we can access the admin site but the public site is not available now.

Mon Jun 26 14:26:04.445479 2017] [fcgid:warn] [pid 970] [client 105.186.156.83:61691] mod_fcgid: stderr: PHP Fatal error: Call to a member function link() on a non-object in /home/cmchobbies/public_html/shop/catalog/controller/error/not_found.php on line 12

this is whats on line 12 :
'href' => $this->url->link('common/home')

can anybody shed any light on this please?
Many thanks
clive