Stolen

Did you have Find My Phone turned on in Settings > iCloud? If so, have you tried logging in to iCloud, click Find My Phone and see if it shows up? If it is turned off or not attached, most likely since you say this happened New Years Eve...it will not show up.

With Find My Phone activated, the iPhone cannot be used without knowing your Apple ID and password...activation lock is a feature of iOS 7.0.4. You can also send a signal from iCloud to disable the iPhone should anyone try and use it.

Report this to your insurance, change passwords associated with any accounts on the iPhone and notiy your carrier in case they offer blacklisting.

Sign into icloud.com with your Apple ID and password if you have activated Find My iPhone. If it is online, you should be able to track it. This is all that you can do other than filing a police report. Apple has no way to track your device for you. Best of luck.