Luckily, Find My iPhone is a functionality that's built right into iOS 6 and 7 and, if enabled, will definitely increase the possibility of finding your phone.
Before you do anything drastic, try calling your phone from another device. If that doesn't help, try the steps below that apply to you. If you're sure your phone is lost forever or stolen and you want to suspend service to the phone, you'll want to remotely erase the phone first and then suspend service. Service has to be active in order for Find My iPhone and any other cloud functions to work consistently.
Of course data has to be enabled on both the phone and the account for most Find My iPhone to work optimally. If your device is able to connect to a recognized Wi-Fi network, Find My iPhone will work without data being enabled, but if the device is unable to make an Internet connection, then it won't work
Find My iPhone is Enabled
Log into your iCloud account, click on Find My iPhone to try locating the phone on a map.
If your phone is not in a location that you recognize, you can put the phone into Lost Mode from this interface. This allows you to set a passcode and display a message indicating that the phone is lost. If the phone is offline when you do this, Lost Mode will take effect once the phone comes back online. Lost Mode is available on iOS 6 and 7.
To delete all of the information on the device, you can remotely wipe it, but once it's erased, Find My iPhone no longer works.
1. Select the device from the Find My iPhone drop down
2. Click Erase iPhone
3. Once erased, click Remove from Account
4. Suspend or deactivate the device's number from your Madstar Mobile account
If you have Find My iPhone enabled on a device running iOS 7 or later this will automatically turn on the Activation lock which keeps anyone else from resetting or reactivating your device with a new number.
Find My iPhone Disabled
If trying to call the phone isn't working and Find My iPhone isn't enabled, then all you can do at this point is to cut off service and ensure that whoever has it can't delete your information from the iCloud.
1. Log into your iCloud account
2. Click on the dropdown next to your name on the top right of the screen
3. Click on your AppleID
4. From the Manage your AppleID page, reset your password
5. Suspend or deactivate the device's number from your Madstar Mobile account
This doesn't keep whoever has the phone from accessing any data on the phone, but it does keep them from being able to delete anything in the iCloud.