I get the following window on loading Lightroom 5.7 -
Lightroom encountered an error when reading from its preview cache and needs to quit.
Lightroom will attempt to fix this problem next time it launches.
I have tried several restarts, without success. Also reinstalled Lightroom; no joy
Using your system browser (Windows Explorer or Finder), open the folder that contains your catalog. In Windows it is in your pictures folder and then in a Lightroom folder. Inside of the Lightroom folder you will find another folder that has the extension .lrdata. It will have the same name as your catalog except for the extension. You need to delete that folder, and then start Lightroom again. Lightroom will generate a new previews folder, and you should be back in business.