I had this issue. It turned out to be a corrupt catalog that was caused by bad ram in a newly built computer.
First of all, in the Lightroom Classic folder, delete any and ONLY any file ending in .lock.
Then try your most recent backup. If that is a backup made by Lightroom Classic, you will find that it's a zip file. Unzip it. Then in the Lightroom Classic folder rename the current catalog that is not opening. After that, copy the unzipped backup to the Lightroom Classic folder. Double click that file to start Lightroom Classic.
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