Some of my events have disappeared/been deleted. How do I get them back?
Agenda does not store any information, it just displays what is in your iOS calendar. So, no information has been lost.
Depending on your iOS settings, the iOS calendar may only have the last month or so of information stored in it. Visit the iOS settings under "Mail,Contacts,and Calendars" and change your syncing period to show "All Events" under "Calendars."