The FBI will not be shutting off your internet but you really should watch where you surf!
This called a ransom virus and most likely indicates you have surfed somewhere you should not have gone in the first place.
Do not click on ads and give your creditcard number to someone offering to fix it!
Your easiest way out of anything like this is to try to initiate System Restore.
If it wont start in regular mode you can initiate it in Safe Mode reached by tapping F8 repeatedly while booting up.
Check your restore points!