How do I find out where the folder is located?
In File Explorer / Mac Finder search your computer for files ending- .LRCAT.LOCK
This should find the file that is 'locking' Lightroom from opening.
Delete any file ending with .LOCK Then try to start Lightroom.
A 'Lock' file is created when Lightroom is operating so that the catalog is protected, and it is removed when you close Lightroom correctly.
If you computer crashes or shuts-down un-expectedly then this 'lock' file can get left behind and stops you re-opening Lightroom.
It is only necessary to delete the .LOCK file. You may also see a .LRCAT-JOURNAL -file that can remain- it may hold your latest editing work in Lightroom before the crash.
Wobertc has provided you correct suggestion.
Also, if you know the location of the catalog, you can directly go to that folder and delete the .LOCK file.
By default, the catalog is stored in the Pictures folder.