There is a place where you can make things hidden. It's called restrictions. If you go into settings general then restriction you can make things that seem to disappear and then will be on again. Maybe that could be it or you moved it to a different page. You cant delete anything that comes built into the phone.
Sorry I thought you ment the app
If you delete the calendar itself the only way to get it back is if you backed up your phone. I have sometimes unchecked a calendar to make it seem like I deleted it but i didnt. You have to go into edit to do anything. If you delete something after you press delete it will triple ask you if you want it deleted.