Find my iPhone

Unless you had Settings > iCloud > Find iPhone enabled on it you could try locating it either via http://www.icloud.com on a computer or Find iPhone app on another iOS device - but that will only work if it's connected to a network.

If you think that it was stolen you should report it to the police (to get its serial number : Find the serial number of your Apple product - Apple Support). You should also change your iTunes account password, your email account passwords, and any passwords that you'd stored on websites/emails/notes etc., and contact the carrier.