start problem Classic

When Lightroom is running and has a catalog open it creates a lrcat.lock file next to the lrcat file. 

This is to ensure there is no other access to the catalog when  he is in use. When Lightroom is shutdown the lock file is automatically deleted.

If Lightroom is not shut down properly this file remains in the folder next to the lrcat file preventing it from opening. When Lightroom is closed go to the folder where the lrcat file resides (normally user name>pictures>lightroom). If there is a file there lrcat.lock delete the file and then try and restart Lightroom . 

Please take a look here: https://helpx.adobe.com/lightroom-classic/kb/lightroom-error-catalog-cannot-be-opened-lrcat-lock.html