Troj/Reveton-Ransomware family consists of computer infections that lock you out of computer unless you pay a ransom. It does this by displaying a lock screen when you login to Windows that pretends to be from a law enforcement agency in your country. In order to access your computer you must submit a MoneyPak voucher, or other payment coupon, to the malware developers and they will then unlock your computer so you can access your Windows desktop again. The messages are fake and you have not actually been locked out of your computer by the government.
Source: https://www.bleepingcomputer.com/virus-removal/family/troj-reveton-ransomware/