Please excuse my poor english.
I work on a Mac Mini with dual screen display. When I launch Lightroom, it displays on my main screen the secondary display (magnifier mode) and I can't access the main Lightroom panel (library, map, print...).
I first updated to the last version, then restarted my computer. Still the same problem. When I switch off one of my screens, I can access the development tool. Then, I choose to disable the second display. When I switch on my second monitor, Lightroom turns back to the secondary display.
How can I reset Lightroom settings to start working again with my two screens?
Sounds like your second, secondary, display is listed as the Primary by OS X.
I ran a Mac for a little over 2 years, notebook with an external monitor connected some times, and IMHO Mac's don't do dual screens all that well. Not as easy as a Windows notebook.