LrC will not open a catalogue

Please take a look into the folder where the catalog file is located if there's a .lock file. If yes, delete it and check again if LR opens the catalog now.

In a next step try is to reset the preferences to the default settings by doing the following steps:

• 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

If this all doesn't help please give us mor information as @dj_paige wrote.

Lightroom Classic won't open a catalogue

By @Botzkah

Why not? What happens? If there is an error message, please tell us the exact unedited word-for-word error message.

Also please state the version NUMBER of your LrC and the version NUMBER of your operating system. We need version NUMBERs and not words like "up-to-date" or "latest".