When the computer is locked by the virus that tells of itself as the message supposedly coming from the FBI there's no way how one can unlock the system using the standard approaches. If you have some antivirus program you will not be able to run it because you simply can't access your start menu or the desktop icons, because your desktop isn't available. If you, however, restart your computer the same scary notice will prevail, no matter how many times you've done this. As you see, the situation some users face because of this virus may be really scary for them, since they don't know the way how their workstation can be unlocked. Moreover, some users tend to believe that the message they've got indeed comes from the FBI.
The very content of the message allegedly coming from FBI reports that some particular user (like you, for example) has been found to perform various illegal activities over the Internet. For example, you can be accused by the malware of downoading, keeping and distributing illegal copies of audio and video files, software, supporting terrorist activities, sending spam to various locations, etc. Also very often users are accused of watching a lot of sinful stuff over the Internet. Without a doubt, receiving such message can be very scary for all users who don't realize that the message they've received is a fake one, not being associated with the FBI at all.
Once the trojan horse performs its successful role in locking the computer with FBI virus warning about your computer being locked, the common desktop background of the computer is being replaced with another one, where the text of the ransomware message is written. The malware instruct users to pay the amount of fine (penalty or forfeit) in favor of the crooks. This is what the word "ransom" actually means. However, users don't realize that they don't actually transfer funds in favor of the government, but into the pockets of the fraudsters, instead. This particular screen locker that says that your computer has been locked by FBI is extremely popular in the United States of America and Canada where GreenDot MoneyPak payment processing system is available and very popular. Thus, the ransomware instructs users to indicate the special voucher or PIN code of GreenDot MoneyPak system in the respective field of the ransomware's message. Also, as an option, sometimes the virus tells users to send such financial information via email onto the address email@example.com. But they don't realize that this email isn't even used in FBI, being totally at the disposal of the crooks.
We hope that after reading the above-mentioned information you will not trust anything contained in the ransomware's notice. Remember that your system is not locked by the FBI, but by the virus makers that prepared and launched such a serious desktop hijacker. Do not pay money in favor of the crooks. In order to unlock your PC we strongly recommend you to follow the removal guide you see in the link below. This will help you restore your common desktop background and the entire computer into the normal, fully-functional condition.
How to unlock your PC from FBI virus warning: