Depending on your operating system, use Windows Explorer or Finder to open the folder containing your catalog. In that folder you will find a file with the same name as your catalog, but with the .lock extension. That file needs to be deleted, and then you will be able to start Lightroom.
Mine doesn't have a .lock extension I don't think. It just has one Lightroom.
Then you need to configure your operating system so that it does show file extensions. They are there. Your system simply isn't showing them.