My site seems to be dead

Just tried to log in to my site and the first strange thing I noticed was an 'Access denied' message when going to the admin page. Then I couldn't login and when I finally logged in with uid1, I still got the access denied message when visiting the admin page.

I emptied the sessions (wrong thing to do?), user_permissions and cache tables, all of which had quite a bit of overhead. Still no result. When I tried to reach the admin page after doing this, I got the following error:

Fatal error: Duplicate entry 'fcfffb08cf7831fd970f8e81737811d6' for key 1 query: INSERT INTO sessions (sid, uid, hostname, timestamp) VALUES ('fcfffb08cf7831fd970f8e81737811d6', 0, '59.144.28.109', 1137339653) in /home/domain/public_html/beta/includes/database.mysql.inc on line 66

Additionally, clean urls seem to have stopped working and I get the typical drupal urls with ?q

It was working perfectly until about 24 hours ago and the only change I added was the 'Who's new' block, which is wildly fluctuating between 2 guests, 24 guests and so on. I am using 4.6.5.

The drupal install is on a directory called 'beta'. I just confirmed that two other drupal installs at the root of my domain and in another directory work perfectly, so the problem is definitely limited to this install.

Any suggestions as to what could be wrong? This is a production site that is less than a week away from being launched. Gives me the jitters.