Update to 3.4.2 failed

I was trying to update from an earlier version of WP (I think 3.3.1, but can’t check now).
Before I did anything I took a back-up of my website.
When I pressed the update button I got the following message:

Fatal error: Call to undefined function get_option() in /home/pooky2/public_html/wp-admin/admin.php on line 32

What should I do?
Should I simply restore the entire site, or will that cause a conflict with the newly installed version of WP? Or is it better to roll-back to an earlier version of WP?