lost my sidebar following 3.5.1. update

Hi there,
my website is: (hope this works as link?)

the error message all pages is :

Fatal error: Call to undefined function dynamic_sidebar() in /www/sites/3b6/8d1/www.qimoving.co.uk/web/wp-content/themes/twentyten/sidebar.php on line 20

In the sidebar were posts with sticky publishing command on them. The posts are now invisible.

I have found the sidebar.php in my admin files under themes, but I don’t know what to do with line 20.

I have also found that I cannot drag widgets into the sidebar anymore.

The error message is:

Fatal error: Call to undefined function dynamic_sidebar() in /www/sites/3b6/8d1/www.qimoving.co.uk/web/wp-admin/includes/widgets.php on line 80

I don’t dare updating twentyten to the latest version, (which might help!?) because I am worried instead of helping it will just cause more havock.

Can anyone help?

Thanks