I've had this in the past as well...you have to re-boot in safe mode (F8 on most pc's)then do a system restore to a point before your trouble started.Once that is done and you restart you should have access to your drive.I ran malware bytes and spybot search and destroy(run them both as administrator),then you can back up and reformat if you wish.This crap is called ransomware and its made to take advantage of people's naivity of their computers,and scare you with the child porn and stuff so if you dont know any better you pay them for the "fix".
Good luck....it seems scary but there are much worse things out there.