How to Change Stolen iCloud Password

It sounds like whoever has your phone hasn't bothered (most likely doesn't know how to) to change the Apple ID and has entered their own contacts in. You should take this information to the police.

Then check if there's a contact labelled "Mum", call the persons mother and tell them that their child is a thief :-)

Best of luck!

You haven't mentioned it, but even if you had your location services disabled and your iPhone was untrackable with "Find my iPhone," you should inform Apple that your phone was stolen. If you bought your iPhone new, then Apple will have record of that iPhone's serial number (and probably MAC address) associated with your name. If the serial number shows up in their systems, either by the thief seeking support from Apple, or by the theif's creation of an AppleID and attempting to reassociate that iPhone to their name, Apple can flag the activity as suspicious, remotely wipe the phone, or more likely, simply disable it. I highly recommend calling Apple support and reporting that your iPhone was stolen.