If you are an icloud user you can take advantage of a feature called "Find my Iphone". "Find my Iphone" is an app that allows you to locate where your Iphone or Ipad is if you think it is lost or someone may have taken it. To set up this service you must do the following:
Go to Settings
Tap on the icloud option
Enter you Apple ID and Password
Turn on Find my Iphone
With "Find my Iphone" now enabled if you cannot find your Iphone or Ipad you can log into your account on icloud.com or use the Find my Iphone app on another Iphone or Ipad to see an approximate location of your device.