You can start by telling us about your system.
What are the errors???
Geoff -- not sure - but I reinstalled the 5.7 I had downloaded and all is OK now. When it happened, my computer froze up as I was installing 5.7. a total freeze - I had to hard kill the machine. When I tried to reboot, it would get as far as starting to populate my desktop icons then go to blue screen. I could only start in safe mode. I restored back to the point that I started to install 5.7. The computer would boot OK, but then I couldnt start 5.7. Here's what I got. Different catalogues wouldnt open. That's when I reinstalled 5.7 and all seems well at this point. Many thanks for getting in touch. Randy Schaeffer
in this case you could use a backup of catalog - depending on your settings LR creates for example weekly a backup of the database...
The work you did since last backup would be lost if you didn't store all changes in the image itself. Virtual copies of your pictures since last backup also will be lost.