Lightroom lässt sich nicht mehr öffnen

Please try is to switch off the GPU support from the Lightroom preferences and check if that helps to fix the issue. 

Go to Lightroom > Preferences > Performance tab > Uncheck "Use Graphics Processor" > Restart Lightroom. 

https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html 

If Lightroom doesn't start correctly the please see "Solution 2" in the document behind the link below. 

Troubleshoot GPU issues | Lightroom Classic (adobe.com) 

Another step is to try to reset the Lightroom preferences. 

https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/ 

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 

Do you have send the crash report to Adobe so that they can check?

If Lightroom is running it creates a .lock file for the opened catalog next to the approriate .lrcat file.

On LRs regulary shut down this file was deleted. If LR crashes the .lock file remains in the appropriate folder.

In this case the .lock file must be deleted manually.

More informations about this, you'll find here: 

The Lightroom catalog cannot be opened because another application already has it opened - lrcat.loc...

If this doesn't help we need more informations about your environment.

Which operating system do you use?

Which version of Lightroom do you use?

Please post the exact version and not only phrases as "recent", "latest" and so on.

Please provide also a copy of your 'System Info'. This can be get from the LrC Help > System Info menu item.There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.

Do you get an error message? If yes, please post the exact message or an screenshot.