Well, it.... What?
That is .Lock (DOT ( . ) lock) and it is in the same folder as the catalog file.
It has the same name as the catalog. so it would be Lightroom 5 Catalog.lrcat.lock. Search for and delete that file.
File name should be Lightroom 5 Catalog.lrcat.lock
When Lightroom opens a Catalog file it immediately creates a file with the same name as the Catalog file that is opened with an extension .lock (.LOCK)
This Lock file prevents any other user from accessing the same catalog file, and is automatically removed when you exit Lightroom normally. However if lightroom closes abnormally i.e. crash or power loss the .lock may remain alongside the catalog file preventing any further use. Deleting the .lock file should allow lightroom to reboot normally.