Lightroom Classic lässt sich nicht mehr starten

Using Windows File Explorer, bring up the folder your catalog is in, and share a screenshot of that info, please.

Unfortunately you haven't given us any detailed information about your issue and your system. Therefore, all proposed solutions are a look into the crystal ball.

Has LR been crashed? If yes, check if there's a .lock file in the folder where the catalog is located.

The Lightroom catalog cannot be opened because another application already has it opened - lrcat.lock (adobe.com) 

Do you get an error message? If yes, please post the exact message word-by-word.

You can also 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