Today I am unable to launch Lightroom 6. The message that comes up is "Lightroom quit unexpectedly- click reopen to open the application again." Nothing happens when I do this. I tried restarting without any success and the same message appears. Operating system is 10.12.5(Sierra), No problems before. Does this mean I have to reinstall LR6? Will I lose backup catalogs? Any help would be appreciated.
Reinstalling Lightroom will have no effect on the catalog you have started. That means that any work that you have done will not be impacted by a reinstall. As to whether the reinstall will fix the problem you are facing, I don't know if that will help or not.