You reinstall LR 5.6 (from your old copy or from Adobe archives), enter your licence number, and find your old 5.6 catalog. Simple
If you haven't got an old LR 5.6 catalog, you can't go back. LR6 irreversibly changes your 5.6 catalog.
FYI, 5.7.1 was the last version of Lightroom 5.
Not so simple,
I have had it on my machine all the time...can't understand why
suddenly, I need to reinstall.
I didn't touch the installation, so, why do I suddenly need to get a
trial version ??
Things go wrong with computers! None of the bits of hardware or programs or OS's are perfect - they are made and written by people. And to add to that, our computers are bombarded with stuff like cosmic rays all the time. A strike from one of those on your memory just wipes out that bit of memory, so you have an error next time your use that bit or save it. Unless you have ECC memory, those hits accumulate.
So no use in crying over spilt milk, just open the fresh bottle!!!