After DB backup and restore: Call to undefined function user_access() in ...includes/menu.inc
Here are few tips:


make sure your Drupal core has all the files in the right place (especially user module),
upgrade your Drupal core to the latest version (drush fr &amp;&amp; drush up drupal)
clear all your caches (drush -y cc all), including your memcached (killall memcached),
rebuild your registry by using Registry Rebuild,
if you're using modules which are implementing drupal_bootstrap() (such as Secure Site), disable them,
try disabling your PHP cache engines (such as XCache, Zend optimizer) to see if that's the problem,
use XDebug and generate a trace file to analyse the code execution,
or use IDE debugger, breakpoint on the failing line and check why the function doesn't exist, 
test it on different environment/machine,
eventually restore your database from the point where it was working before.