There are hundreds of sites that popup these fake warnings. There are hundreds of topics on these forums alone about them. They are ALL fake messages from scammers who in fact found absolutely nothing on your Mac and charged you a ridiculous amount to money to "fix" nothing.
Since you have no means of paying them (unless you gave them a checking account number or similar), then ignore any other correspondence you get from them.
There's also a very good chance they installed malware on your Mac. Like a keylogger to capture those passwords or other personal info. What you have to do at this point is backup personal info such as your email, photos and such, then erase the drive and restore it to a point before you allowed them access.
If you have no safe backup to restore, then you must backup your personal data, erase the drive and reinstall everything from scratch. Then manually restore your backed up files.