Fatal error: Class 'CI_Controller' not found

Hi, I'm a completely new CodeIgniter user, and I came to this error when calling a function : 

Fatal error: Class 'CI_Controller' not found in .........

the URL is http:/-------------.com/join/confirm/?code=ec5dd81/

I assume it has something to do with the question mark in the query, because if I change it to confirm/code/ec5dd81/ it gets through, but then messes up other things, so I need this to work with the question mark.. It worked before, I don;t know what has changed, didn't do any upgrade..

my htaccess file is like this, if it makes any difference>:

RewriteEngine On

AddDefaultCharset UTF-8

Options -Indexes

Options +FollowSymlinks



RewriteBase /



RewriteCond %{HTTP_HOST} !^(.*)\.

RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]



RewriteCond %{REQUEST_FILENAME} !-f

RewriteCond %{REQUEST_URI} !(.*)/$

RewriteRule ^(.*[^/])$ $1/ [L,R=301]



RewriteCond %{REQUEST_FILENAME} !-f

RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule ^(.*)$ index.php?/$1 [QSA,L]





ErrorDocument 404 /index.php?do=error
ErrorDocument 500 /index.php?do=error



Please help, I'm going desperate here..
Thanks
Miro

See the CodeIgniter Documentation below:

CodeIgniter URL's

What did you Try? What did you Get? What did you Expect?

Joined CodeIgniter Community 2009.  ( Skype: insitfx )