Virtuoso, by default, creates a file with the same name as the Database file but with the extension of ".lck". This file exists when the Virtuoso server is running to prevent it starting multiple times using the same parameters, and is automatically removed by the Virtuoso server when it shuts down. If the Virtuoso server is not shutdown cleanly due unexpected machine or server failure etc. the lock file will not be removed and the server will fail to restart as the lock file will still be in place and the server will assume an instance is already running. In which case the lock file, located in the directory containing the database files, needs to be manually removed before starting the Virtuoso server.