Reveton is a ransomware that pretends to be from the police.
After the trojan successfully infects a machine, it will prevent the user from accessing the Desktop and will display a fraudulent message alleging that the system was locked by a local law enforcement authority; the specific authority mentioned varies depending on the affected user's location, though most of the samples we have seen mainly mentioned various European authorities.
This malware appears in many countries, pretending to be the police. The virus goes by many names depending where the user is from. examples: FBI MoneyPak (United States), Metropolitan Police (United Kingdom), Celas (Russia), and Canada Police Association (Canada).
Malwarebytes is known for its best job at removing Reveton.
In order to get rid of Reveton, boot into Safe Mode or use System Restore and remove the program.