LcR don't start.

Please check if there's a .lock file next to the LR catalog in the appropriate folder.

If there's such a .lock file it 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.lock (adobe.com)

If this doesn't help you can try to reset the LR preferences to the default settings.

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

The next is to try the following steps:

• Close Lightroom
• Backup the folder : C:\Users\xxxx\AppData\Roaming\Adobe\Lightroom to a save location and then rename it to LightroomOld
• Restart Lightroom

Another try is to hold the Ctrl-Key while starting LrC. This opens a catalog selection dialog.

Select the appropriate catalog and let check it – note the checkmark to check its integrity. If that does not help, try to create a new one and see if this catalog then works. You could then try to import your current catalog into the new one.

If this already doesn't help, provide 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.

What has been changesd on your system before the issue occur?