This error can occur for a range of reasons in Moodle, but can often be resolved by a simple clearing of the caches... manually.

Find the 'lock' folder within your moodledata directory and manually remove all the contents via FTP.

This should resolve the error, or at the very least allow you to regain access to the Moodle site to begin the process of debugging the cause of the error.

Sometimes it's simply due to a corrupt file within the moodledata/lock directory, and by manually purging this folder you may find that the error eases up and is relieved.