My calendar app that came with the iphone4s disappeared? How can I restore it?

Plug your phone into your computer and pull a back up if you have on from itunes.

Also verify you phone is using the correct software 6.1.3 or 6.1.2 Iphones are known for "losing" things when software becomes out of date.

If that doesn't work then I'd advised doing a a factory reset. Make sure you back up your phone to itunes first.

Does anything else not work or acting strange?

I also had this happen to me. I just did a spotlight search, and found that it had moved to an odd folder, and was the "10th" in the folder, so it was set on a second page, so I couldn't see it!