Recently, I went to export my photos from LR and the following message came up, "The Lightroom Catalog named "Lightroom 5Catalog cannot be opened because another application already has it opened. Quit the other copy of Lightroom before trying to launch."
Below that paragraph are 3 boxes: "Chose a different Catalog," "Continue" & "Exit"
And, LR won't open now.. Any suggestions?
Look in the folder the catalog is stored in for a file with a .lock extension. Delete that .lock file.
Make sure LR is closed when you do that.
With Windows Explorer, go to the folder (usually named Lightroom) that contains your Lightroom catalog (usually inside the My Pictures folder).
Delete the file with the .lock extension. Do not delete anything else.
Now launch Lightroom.
The lock file is present when Lightroom is running to prevent other users from accessing the catalog, and is deleted when you close Lightroom. If Lightroom isn't closed down properly, it may not be deleted, and Lightroom will not launch,