Problem when opening

 An Error message like "....I get a small message Ligthroom-Catalogus openen...." suggests that there is a .LOCK file still existing that does not allow the catalog to open.

Search with File Explorer for a file with file-type .LOCK and delete it. Do NOT delete anything else.

https://helpx.adobe.com/au/lightroom-classic/kb/lightroom-error-catalog-cannot-be-opened-lrcat-lock.html

First try to reset the preferences of Lightroom Classic:  How to reset Lightroom Classic preferences (adobe.com) 

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

If this doesn't help restore the catalog form the latest backup.

If this already doesn't help try to create a new catalog. Hold the Ctrl-Key while starting LrC. This opens a catalog selection dialog.

To create a new one by clicking on "Create a New Catalog" and see if this catalog works. If yes, you could then try to import your current catalog into the new one.

If this all doesn't help we need more inforamtions about your system.

What has been changed on your system before the error occur? Installation of updates, patches and so on.