Zend Optimizer+ error
The very same thing happened to me after transferring my website to another hosting company.

You must run update.php.

Since I was logged out, the way I resolved this is as follows: I typed in the url box www.example.com/update.php (www.example.com being your site's name. If it's on your local machine, I guess it is something like localhost/update.php.)

If your settings.php file is properly configured, you should get an access denied message.

In that case, go to the directory (sites/default) and open the settings.php file. Look for the line $update_free_access = FALSE; and change it to TRUE. 

Run update.php again. You will find a link at the bottom of the result page sending you to the front page. Click on it and your site is back in business. :)

CAUTION. Very important: Go to your settings.php file again and change the TRUE back to FALSE.