Strange problem with iPhone 8.0, when I press on the slideshow button for an event, nothing happens, then when I click again, the screen goes blank like it is trying to do a slideshow and then returns to the main iPhoto screen. Nothing on the web on how to fix, but feels like either the plist (preference list) or the iPhoto library is corrupt. Easy to do with 2,000 photos!
Some fixes are noted on discussions.apple.com including deleting manual the preference list, it is in /yourusername/Library/Preferences/com.apple.iPhoto.plist and also holding the Option key down when starting iPhoto and then select Choose Library to help it find your library of iPhoto or try to backup and rebuild. Hold down the Apple and the option key both down to rebuild.