When I start in lightroom (latest) and edit a photo in PS (latest), when I switch back to LR, there is only a limited set of menus in the menu bar and no main LR window. I have to open the catalog to get it back. I'm on a mac, and the main window comes back almost instantly when I open the catalog, but if I use the F12 key to see all the windows on the computer, it's not there. This is a PITA. I'm in Sierra 10.12.2 on a mid 2012 15" MacBook Pro, full memory and lots of disk space. Thanks for any help.
If you have not changed anything, as far as the viewing options then something must have gone wrong with the preferences. Try reset the Lightroom preferences and check if that helps, refer: Preference file and other file locations | Lightroom CC and 6