Reminders just stopped working for me in OS X 10.9. I opened it from the dock. The menu opens but no window.
I tried disabling reminders in my iCloud setting, then reactivating and syncing with iCloud. The window opened but without the sidebar; I could not see my lists. Weird.
So I went to /Library/Containers/com.apple.reminders/ and used Time Machine to restore this folder.
Reminders now works again. How did this happen? Reminders has gotten better but it still has a few bugs. It doesn't always sync properly; now it stopped working altogether. Hmmm.