You were entering text in an input box and then exited before saving: why would you expect there to be a way of recovering stuff that was never saved?
Just a warning: if you delete files in android they are gone. There's no system waste/recycle bin, delete means delete. I thought I should mention this since it's sort of related and it's more common to assume that file deletion is reversible.
As for the app "just closing itself", that could be an app crash or an accidental button press (e.g. inadvertently giving a long press to the back button while typing). In my experience app crashes are very rare, but I don't know what phone or what calendar app you are using. It used to be that when an app crashed you would get a message on the screen, but one of the many bad ways Google have followed Apple is in removing such messages (there used to be a perception that apps crashed more often on android than iOS, when in fact the reverse was true. But the reason was that android told you the app had crashed, while iOS just silently returned you to the home screen, and many people didn't realise that this meant the app had crashed. And now Android does the same, so you can't tell whether it crashed or you accidentally closed it).