Lightroom Classic won't start

Please try 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/lightroom-gpu-faq.html#troubleshooting

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 second link.

Troubleshoot GPU issues | Lightroom Classic (adobe.com)

So, this one has me fairly stumped, but some possibilities

1. A corrupt preferences file. The Preferences file can got corrupted leading to buggy behaviour. The preference file is normally deleted during uninstalls, nor is it normally replace during installs. It is first created upon the first start of LrC. Sometimes this file needs to be reset, or as Adobe now appears to be calling it, restored. This does not involve changing the various settings in preferences, although the procedure will revert those to defaults (many or all), it involves, basically deleting a very specific files, Upon restart of LrC, it gets recreated from scratch.

As you do not appear to be getting as far into the LrC startup as seeing the splash screen, this may not be the issue. A corrupt preference file, when it causes a failure of LrC to start usually shows up in the splash screen when the preference file is being read (and it never completes that reading, Following is the screen (perhaps a bit dated) that you would see:

Notice the "reading preferences..."

By the way, you do not get to this point, correct?

In the link below the method(s) to accomplish this is given. Preferred way is withing Lrc, but a manual way is provided. Within LrC is prefered as you can prepare for thee loss of links to presets, plugins, templates, preferences settings, etc. But a simple manual way to delete the file is provided.

Make sure to remember where your catalog is placed, If LrC starts up, it will not at that point know where it is, and may make a new catalog. Also, upon restart of LrC, it will act like a new install (welcome screens, tip screens)

I am hesitant to suggest this will work, but..

https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html

2. Perhaps rolling back LrC to the previous version will get you back up and running (to then be followed by figuring out what the heck went wrong) . This is probably nothing to do with the v12, but with something kaput on your computer, maybe some jink left behind, maybe a bad install. Do not know.

https://www.lightroomqueen.com/roll-back-update-previous/

3. A bad installation of the Upgrade, you can try uninstalling :LrC, then reinstalling. Basically as accomplished in a roll back, but trying the v12 install over agin instead of the v11.5

4. A very very bad install that fouled who knows what up, requiring a complete uninstall of LrC (hopefully not PS, ACR, etc). This would not be a simple click on uninstall (as you would be doing in #2 above) in the Adobe Creative Cloud Desktop App, or a uninstall within the Windows OS (/settings/Apps)  but would involve a removal tool provided by Adobe.

This is not for the faint at heart.

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Hopefully other mebers will reply.

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And it may be past time to give in and call Adobe. (or start a chat)

https://helpx.adobe.com/creative-cloud/kb/cc-cleaner-tool-installation-problems.html

As for the inquiry on modules, have you ever used a Logitech mouse and keyboard, and If so, does it use a app called Logitech Options to control the mouse?

Just looking for one suspicious culprit

Bring up your copy of the Adobe Creative Cloud Desktop App.

• Are you signed in (top right)?
• When you mouse hover over Lightroom Classic, does v12 show up as installed? Or what version is it?

On the Windows PC, in Settings, in System, then About, look at the info as follows, reply on what is stated:

On the Windows PC, right click on the Start icon (bottom right) select Run, type in Control, Select Device manager, select Display Adapters, right click on the adapter, select Driver, what is shown? (GPU make/model and driver version

In the Windows OS, Use the Windows File Explorer, Navigate to your user account/settings, in that select AppData, in that select Roaming, then Adobe, then Lightroom, and finally Modules. Take a screen capture of that and p[ost in a reply (mind you suspect would be LogiOptions, not much more, that one is a big pain)

In that folder you posted the screenshot of. What happens if you double click on the catalog?

For the members that will not look at attachments, following is the authors screenshot of the folder

Author states after deleting that lock file, LrC still will not start.

Check the manufacturer's website for a driver update for your GPU card.

How about a screenshot of that loading symbol, full screen probably. 

Please provide some basic info. See the screen capture.