The only security against ransomware is maintaining current backups of all your critical data on an external resource that is not generally connected to your computer (an external USB hard disk or a Cloud resource such as Carbonite or CrashPlan).
Because of the way that recent ransomware products are continuously evolving, I'm not sure that "prevention" is possible. Mapped network drives have been a target as well as even things like Dropbox.
The typical attack vector seems to be email attachments so, normal prevention still applies (don't open them), but once infected the only way to get your data back is from a backup that was not connected to your computer at the time of the infection.
Even paying the Ransom is no guarantee.
AND
I don't believe that bitlocker or any other encryption software will or is intended to protect you against any malware infection.