I am using Safari version 9.0.3 on Mac OS X El Capitan version 10.11.3. I had dozens of Safari windows and tabs open when it became unresponsive. In the past I could force quit and then the session would automatically restore at the next launch of Safari. However, this time Safari was still unresponsive after force quitting and relaunching (the whole system was having difficulty at this point), so I force quit Safari a second time. Now the last session windows are gone, and the history menu option to restore it is greyed out.
In an earlier version of the system a few years ago I used to manually restore Safari sessions by simply replacing the ~/Library/Safari/LastSession.plist file with a backup. That is no longer working. I have also tried restoring ~/Library/Preferences/com.apple.Safari.plist and deleted the ~/Library/Saved Application State/com.apple.Safari.savedState folder since it did not exist before the issue; I might have caused creation of the folder by fiddling with the General pane System Preference setting to close all windows when quitting an app (I unchecked and checked it back on), and the Safari > Preferences... > General > Safari opens with: setting (changed from "A new window" to "All windows from last session" and then back again). Any further ideas of how to restore the session? The History option to restore the session is greyed out no matter what files I restore from the backup.