Classes/Console/Command/BackendCommandController.php
public function unlockCommand()
    {
        if (!@is_file(PATH_typo3conf . 'LOCK_BACKEND')) {
            $this->outputLine('<warning>Backend is already unlocked.</warning>');
            $this->quit(0);
        }
        unlink(PATH_typo3conf . 'LOCK_BACKEND');
        if (@is_file(PATH_typo3conf . 'LOCK_BACKEND')) {
            $this->outputLine('<error>Could not remove lock file \'typo3conf/LOCK_BACKEND\'.</error>');
            $this->quit(2);
        } else {
            $this->outputLine('<info>Backend lock is removed. User can now access the backend again.</info>');
        }
    }
public function unlockCommand()
    {
        if (!@is_file(PATH_typo3conf . 'LOCK_BACKEND')) {
            $this->outputLine('<warning>Backend is already unlocked.</warning>');
            $this->quit(0);
        }
        unlink(PATH_typo3conf . 'LOCK_BACKEND');
        if (@is_file(PATH_typo3conf . 'LOCK_BACKEND')) {
            $this->outputLine('<error>Could not remove lock file \'typo3conf/LOCK_BACKEND\'.</error>');
            $this->quit(2);
        } else {
            $this->outputLine('<info>Backend lock is removed. User can now access the backend again.</info>');
        }
    }
Unlock backend

Allow backend access again (e.g. after having been locked with backend:lock command).
@see typo3_console:backend:lock