Lightroom Classic startet nur noch als Hintergrundprozess

In a first step try resetting the preferences of Lightroom Classic:  https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html

Backup your preferences prior to resetting the preferences: 

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

If this doesn't help try the following steps:

• Close LrC if open (if the UI isn't visible shut down LR via Taskmanager)
• Locate the file named "metrics.json"
  File Locations: 
  Mac: /Users/<username>/Library/Caches/Adobe/Lightroom/metrics.json
  Win: C:\Users\<username>\AppData\Local\Adobe\Lightroom\Caches\metrics.json 
• Remove or rename the file

Check if Lightroom starts normally.

Please remeber: The folder AppData is normally hidden, so make sure that hidden items view is enabled in the Windows Explorer.

If this also doesn't help, we need more informations about your system.

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.