Make a note of your LR Catalog location—what folder it’s in and what its name is. If you don’t know where it is look in LR / Prefs / Catalog Settings / General and it should be listed.
Go to the folder where you catalog is and delete the file with a .lock extension.
Assuming that you don't have the catalog open in another copy of lightroom, using your OS file browser:
In the directory with your *.lrcat file (the catalog) there is probably a matching *.lock file.
If there is a *.lock file with the same name, delete the *.lock file.