It sounds like you clicked on something that installed a fake antivirus program on your computer. These trick you into thinking your computer is infected so you pay money to activate the program so it can clean up the viruses it claims to have found.
If you are lucky you can boot into Safe Mode and uninstall the antivirus program from Add/Remove Programs or Programs and Features but many times it wont be an option because it doesn't want to let you remove it.
You can run a free program called ComboFix which does a good job of getting rid of most of these types of infections. Since you cant get into your computer you can try to boot up into Safe Mode to run the program.