Error number 1064 CodeIgniter

An error is popping up when I log into my system:

  

Error Number: 1064

     

You have an error in your SQL syntax; check the manual that   correspond to your MySQL server version for the right syntax to use   near 'SET last_activity = 1445537033, user_data =   'a: 5: {s: 9: \ "user_data \"; s: 0: \ "\"' at line 1

UPDATE SET 'last_activity' = 1445537033,
           'user_data' = 'a:5:{s:9:\"user_data\";s:0:\"\";s:7:\"user_id\";s:1:\"1\";s:9:\"user_nome\";s:15:\"Sergio Henrique\";s:10:\"user_admin\";s:1:\"1\";s:11:\"user_logado\";b:1;}'
WHERE 'session_id' = 'fa6f6e8bb9106a277ff5373f8301cac1'

     

Filename: libraries / Session.php

     

Line Number: 306

It was working before, what to do?