If the phone is stolen and the app findmyiPhone is connected or you don't know the iCloud password you will never be able to use this phone, as a password is needed to get past the activation part of the phone when factory restored, then it is a paper weight or sell it for parts on ebay, you'll get maybe 60$
Connect the iPhone to your computer and launch iTunes
Turn the iPhone off (hold down the power button at the top of the iPhone)
Hold down the sleep/power button and home button together for exactly 10 seconds, then release the power button
Continue to hold down the Home button until a message appears in iTunes telling you an iPhone in recover mode has been detected
When you are in DFU mode your iPhone screen will be completely black
Restore
Good luck bud