Uncaught exception thrown in shutdown function.

PDOException: SQLSTATE[HY000]: General error: 10 disk I/O error: DELETE FROM {semaphore} WHERE (value = :db_condition_placeholder_0) ; Array ( [:db_condition_placeholder_0] => 13377859585ce351a26fd152.04691197 ) in lock_release_all() (line 269 of /local/prj.noc.ac.uk/www/includes/lock.inc).