The virtuoso.lck file is a special file which is created when the Virtuoso server is started. The presence of this file indicates that Virtuoso is running, and it is generally deleted when the Virtuoso server is shutdown normally. Howver, if the Virtuoso server or host machine shutdowns unexpectedly, virtuoso.lck remains and has to be removed manually before resuming startup.
Note, on Mac OS X when attempting to start the Virtuoso server with the "virtuoso.lck" already in place using the Apple start script provided will result in the error "Detected stale lock".
In which case if sure the Virtuoso server is not already running the file can be removed manually from "/Applications/OpenLink Virtuoso/Virtuoso 6.2/database/virtuoso.lck".