It’s very likely you are experiencing a Ransomware attack. As a result of opening an infected email or an infected website, a virus or malware is downloaded to your computer which then encrypts the contents or locks the computer and prevents it from starting up. The computer then displays a message which demands payment of Bitcoins, an online currency, in order to obtain a release code to gain access to the computer again. Be careful about the sites that you connect to and if you don’t recognise the sender of an email, don’t open the attachment. Make sure you have up to date anti-virus and anti-malware protection installed and running on your computer.