Lightroom Classic startet nich

The first thing that you should try is to reset the Lightroom preferences to the default settings.

Reset Procedure:

• Close Lightroom.
• Windows: Hold down [Alt]+[Shift] while restarting Lightroom.
  Mac: Hold down [Opt]+[Shift] while restarting Lightroom.
• Overwrite the Preferences when prompted by the dialog.
• Close Lightroom.
• Restart Lightroom.

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

Another 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)

Have you tried to open LR with another or new catalog?

Press and hold the Alt key (Option key on a Mac), then launch Lightroom. Keep the Alt- or Option key pressed until a dialog appears with the option to create a new catalog. Choose another catalog or create a new one.

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

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.